Add PEP 561 marker, fix docstring formatting and make MotionType easier to access

[michaelosthege]

• PEP 561 describes how packages can tell typecheckers that type information is available. The easiest way is to add a py.typed marker.
• Some docstring formatting was fixed (numpydoc style)
  • Empty line after the summary line
  • No : at the end of headings
• CRIController.MotionType was moved to module level
  • Exposed as public API via __init__.py because it's needed to call CRIController.set_motion_type
  • CRIController.MotionType = MotionType monkeypatch to maintain backward compatibility