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
Can you write this instead here?
```
Quantity([1, 0.1], 'cm^-3')
```
If it works I would find it a bit simpler to read.