generate pytask from string and string from pytask (#188)

* pytask creation from bytestring: saving work

* pytask now works

* documentation and tests

* linting

* Lint and fix test case

src/common/lib/python/common_extension.h

@@ -39,6 +39,9 @@ PyObject *PyObjectID_make(object_id object_id);
 
 PyObject *check_simple_value(PyObject *self, PyObject *args);
 
+PyObject *PyTask_to_string(PyObject *, PyObject *args);
+PyObject *PyTask_from_string(PyObject *, PyObject *args);
+
 PyObject *compute_put_id(PyObject *self, PyObject *args);
 
 PyObject *PyTask_make(task_spec *task_spec);