Implementation of GS-based Invariant System Mode Status Requesting and Returning

[chgio]

Following up on #14, *-Mode-Status-Report-Ts could be requested and collated into a System-Mode-Status-Report-T upon reception of an appropriate TC from the GS.

After completion of #14, this shouldn't need anything more than a couple of new interfaces from and to the gs_gui and a new system_mode_status_request_tc_handler component within the OBC.

On the other hand, it would offer the very convenient option for a ground operator to just query the system about its actual mode without commanding any changes.