Implement CSCP in Python and refactorize command handling in Satellite

This MR implements CSCP, adds a CommandReceiver class that handles incoming commands and refactorizes the Satellite to use this. It also introduces a new base class for Satellite communication components such as CSCP, heartbeats, CHIRP and log/stats. This provides the basic infrastructure for e.g. thread handling and makes the corresponding classes much leaner. They can then be used as mixin-classes to the Satellite class.

While especially the latter is a WIP, this MR would be ready to be reviewed, discussed and merged while the refactorization continues.