Manage missing orbits for local spacecraft actor.

# Issue
## Problem Description
At the moment, if the local spacecraftActor tries to run an activity when an orbit is not assigned, a not clear error is raised.
## Expected Behavior
An appropriate error shall be raised suggesting to add "ActorBuilder.set_orbit(...)" when sim.perform_activity(...) is called, if the actor is a spacecraftActor missing of activity.
## What Needs to be Done
If an actor is a SpacecraftActor, when sim.perform_activity(...) is called, it is necessary to check if an orbit is assigned and, otherwise, raise an appropriate error.
## How Can It Be Tested or Reproduced
Try the example in the README on "How to deal with PASEOS activities" and comment the call to the "ActorBuilder.set_orbit(...)" function., before launching the activity.