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
Again ... no need to specify `yr` for `t` input because it's a quantity (I'll stop mentioning that now ... I see another case below where this can be removed).