* [rllib] Separate optimisers for DDPG actor & crit.
* [rllib] Better names for DDPG variables & options
Config changes:
- noise_scale -> exploration_ou_noise_scale
- exploration_theta -> exploration_ou_theta
- exploration_sigma -> exploration_ou_sigma
- act_noise -> exploration_gaussian_sigma
- noise_clip -> target_noise_clip
* [rllib] Make DDPG less class-y
Used functions to replace three classes with only an __init__ method & a
handful of unrelated attributes.
* [rllib] Refactor DDPG noise
* [rllib] Unify DDPG exploration annealing
Added option "exploration_should_anneal" to enable linear annealing of
exploration noise. By default this is off, for consistency with DDPG &
TD3 papers. Also renamed "exploration_final_eps" to
"exploration_final_scale" (that name seems to have been carried over
from DQN, and doesn't really make sense here). Finally, tried to rename
"eps" to "noise_scale" wherever possible.
