add basic structure for task priorities (TODO)

pyrobolearn/priorities/README.md

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+## Priority Tasks
+
+In this folder, you will find the code for priority "tasks". The "tasks" defined here are different from the tasks defined in the `pyrobolearn/tasks` folder which defines robot learning tasks. The tasks defined here can be more seen as "constraints"; for instance, the constraint for the robot to maintain its balance (i.e. have its center of mass above the support polygon), the constraint for the robot to have a certain pose, the constraint for the robot's end-effectors to track a certain trajectory, the constraint for the robot to not have collisions between its links, the constraint for the robot to respect the equation of motions, etc. In the robotics community field, these are known as "tasks" and concepts around them such as the stack of tasks [2] have been defined. In *pyrobolearn*, we keep a more abstract definition of a task, which is also probably more related to what people have in mind when talking about a robot performing a certain task.
+
+Most of these "tasks" are represented as a constrained optimization problem, where the "task" consists to minimize a certain objective function while respecting certain equality and inequality constraints. This is the reason why they are not called "constraints" to avoid the confusion with the (inequality and equality) constraints defined in the optimization problem. Most of the time, they are formulated as quadratic programming (QP) optimization problem [1]. Priority tasks are divided between kinematic and dynamic tasks, where the former only takes into account position and velocity information, while the latter also include dynamic information (forces and torques applied on the various bodies). The variables that are thus optimized by the optimization problem depends on the type of problem (kinematic or dynamic) we are dealing with. In the case of a kinematic task, the variables are often the joint (or end-effector) positions and/or velocities, while in the dynamic case, the variables are the joint accelerations and the (reaction) forces applied on the robot. 
+
+Priorities can be divided into two categories: soft and hard priorities.
+* Soft priorities: each objective function is weigthed by an importance weight where higher weights mean that we give more importance to the corresponding objective function. For instance, we might have a humanoid robot with two arms where each arm has to follow a specific trajectory and where we give the same importance to both "tasks".
+* hard priorities: the most important constrained optimization problem is first solved, and then the next most important one is solved with an additional (optimization) constraint that the solution has to be in the solution space of the previous one. For instance, it is more important for a humanoid robot to maintain its balance than to follow perfectly a trajectory with its end-effector. This way of putting "tasks" on top of each other is known as the stack of tasks in the robotics community [2].
+
+Soft and hard priorities can be mixed together as done in the following C++ framework [3].
+
+The code presented here (and the architecture) is partially inspired by [3], but we decouple it from its tight coupling with other frameworks/middlewares (i.e. superbuild,  XBotControl, ROS/Yarp), write it in Python using optimization libraries (that are generally written in C++ and provide Python wrappers), and provide kinematic and dynamic "tasks" as well.
+
+In what follows, to avoid confusion with the vocabulary used in the robotics community, we will keep the notions of "tasks", (optimization) "constraints", and "solvers". The solvers can be found in the `pyrobolearn/optimizers` folder.
+
+
+## References
+
+1. Quadratic Programming (Wikipedia): https://en.wikipedia.org/wiki/Quadratic_programming
+2. "A Versatile Generalized Inverted Kinematics Implementationfor Collaborative Working Humanoid Robots: The Stack of Tasks" ([code](https://stack-of-tasks.github.io/)), Mansard et al., 2009
+3. "OpenSoT: A whole-body control library for the compliant humanoid robot COMAN" ([code](https://opensot.wixsite.com/opensot), [slides](https://docs.google.com/presentation/d/1kwJsAnVi_3ADtqFSTP8wq3JOGLcvDV_ypcEEjPHnCEA), [tutorial video](https://www.youtube.com/watch?v=yFon-ZDdSyg), [old code](https://github.com/songcheng/OpenSoT)), Rocchi et al., 2015
+4. "Robot Control for Dummies: Insights and Examples using OpenSoT", Hoffman et al., 2017
+

pyrobolearn/priorities/constraints/README.md

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+## Constraints
+
+In this folder, we define the most common inequality and equality optimization constraints used in robotics for priority tasks. Several of them were provided in [1].
+
+Constraints include joint limits, joint velocity limits, collision avoidance, and others.
+
+References:
+1. "OpenSoT: A whole-body control library for the compliant humanoid robot COMAN" ([code](https://opensot.wixsite.com/opensot), [slides](https://docs.google.com/presentation/d/1kwJsAnVi_3ADtqFSTP8wq3JOGLcvDV_ypcEEjPHnCEA), [tutorial video](https://www.youtube.com/watch?v=yFon-ZDdSyg), [old code](https://github.com/songcheng/OpenSoT)), Rocchi et al., 2015
+2. "Robot Control for Dummies: Insights and Examples using OpenSoT", Hoffman et al., 2017
+

pyrobolearn/priorities/tasks/README.md

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+## Tasks
+
+In this folder, we define the most common objective functions (aka "tasks") used in robotics for priority tasks. Several of them were provided in [1].
+
+Tasks include cartesian CoM tracking, cartesian end-effector position tracking, postural positioning, and others.
+
+References:
+1. "OpenSoT: A whole-body control library for the compliant humanoid robot COMAN" ([code](https://opensot.wixsite.com/opensot), [slides](https://docs.google.com/presentation/d/1kwJsAnVi_3ADtqFSTP8wq3JOGLcvDV_ypcEEjPHnCEA), [tutorial video](https://www.youtube.com/watch?v=yFon-ZDdSyg), [old code](https://github.com/songcheng/OpenSoT)), Rocchi et al., 2015
+2. "Robot Control for Dummies: Insights and Examples using OpenSoT", Hoffman et al., 2017
+