Galaxy modelling code revision

Revision of the galaxy source modelling code, including the following changes:
+ Implementation of the spatial and velocity distribution models as `astropy.modeling.Fittable1DModel`
+ Use of `astropy.units.Quantity` throughout the code
+ Added tests
+ Added minimal docs
Or maybe simply not add it to `__all__` so that it doesn't show up in the docs, i.e. it's a "hidden function" and it can stay here?