This PR fixes #530 and should be merged after the firmware PR #1560 is merged. It adds a supervisor subsystem for reading the supervisor states.

Specifically:

• read_bitfield() returns the full bitfield value.
• read_state_list() returns a list of all active states.
• All the individual state checks return True if they're active and False if they're not. can_be_armed, is_armed, is_auto_armed, can_fly, is_flying, is_tumbled, is_locked, is_crashed, active_hl_control, finished_hl_traj, disabled_hl_control

Every time we want to read a state, the entire bitfield is fetched. To avoid sending CRTP packets too frequently, the bitfield is cached for 0.1 seconds.

Also, 2 simple examples are added; 1 for reading the supervisor states and 1 for flying using the state information.

Update:
The supervisor commands (send_arming_request and send_recovery_request) are also moved from platformservice.py to supervisor.py. The user can still use the old commands (e.g. scf.cf.platform.send_arming_request(True)) but they'll get a deprecation warning.