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
I find such very long formulas very hard to read.
Python makes temp variables anyways, so without performance hit you can use temp variables (or split the expression into more lines if you don't like temp variables)