create `constellation` namespace and submodules

This MR separates the Python framework from any 3rd-party Satellite implementation by introducing a new constellation namespace with two submodules:

• core for framework classes
• satellitesfor (user) Satellite implementations

This means that imports would look like this: from constellation.core.satellite import Satellite gets the (base) Satellite while from constellation.satellites.caenhv import CaenHVSatellite gets you the user Satellite.