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Cannot sample orbits around the Moon

See original GitHub issue

Code to reproduce:

In [1]: from poliastro.bodies import Moon                                                                                               

In [2]: from poliastro.twobody import Orbit                                                                                             

In [3]: from astropy import units as u                                                                                                  

In [4]: o1 = Orbit.circular(Moon, 100 * u.km)                                                                                           

In [5]: o1.sample()                                                                                                                     
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
~/Development/poliastro/poliastro-library/src/poliastro/frames.py in get_frame(attractor, plane, obstime)
    254     try:
--> 255         frames = _FRAME_MAPPING[attractor]
    256     except KeyError:

KeyError: Moon (☾)

During handling of the above exception, another exception occurred:

NotImplementedError                       Traceback (most recent call last)
<ipython-input-6-5515a447cc0b> in <module>
----> 1 o1.sample()

~/Development/poliastro/poliastro-library/src/poliastro/twobody/orbit.py in sample(self, values, min_anomaly, max_anomaly, method)
   1042 
   1043         time_values = self._generate_time_values(nu_values)
-> 1044         return propagate(self, time.TimeDelta(time_values), method=method)
   1045 
   1046     def _generate_time_values(self, nu_vals):

~/Development/poliastro/poliastro-library/src/poliastro/twobody/propagation.py in propagate(orbit, time_of_flight, method, rtol, **kwargs)
    225     # If the frame supports obstime, set the time values
    226     kwargs = {}
--> 227     if "obstime" in orbit.frame.frame_attributes:
    228         kwargs["obstime"] = orbit.epoch + time_of_flight
    229     else:

~/Development/poliastro/poliastro-library/src/poliastro/twobody/orbit.py in frame(self)
     99         """
    100         if self._frame is None:
--> 101             self._frame = get_frame(self.attractor, self._plane, self.epoch)
    102 
    103         return self._frame

~/Development/poliastro/poliastro-library/src/poliastro/frames.py in get_frame(attractor, plane, obstime)
    256     except KeyError:
    257         raise NotImplementedError(
--> 258             "Frames for orbits around custom bodies are not yet supported"
    259         )
    260 

NotImplementedError: Frames for orbits around custom bodies are not yet supported

Perhaps instead of being so strict, we could raise a warning and “just” return the CartesianCoordinates. And in that case, perhaps poliastro.twobody.propagation.propagate should ignore the frame, and just produce coordinates:

https://github.com/poliastro/poliastro/blob/1d366845e13d7bac59cdd11a3178fe2126d46827/src/poliastro/twobody/propagation.py#L225-L240