Add Python helpers to make life easier.

- Helper for getting all active controller managers.

- Helper for determining if a namespace contains the controller manager ROS API.

- Helpers for filtering the output of the 'list_controllers' service by
  type, name, state, hardware_interface and claimed resources.
Merge pull request #165 from pal-robotics/cm-msgs-utils

[controller_manager_msgs] Add Python helpers to make life easier.