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
Assign `1.1` to a variable, or maybe even write this?
```
spectral_index = 2.1
```
and then use it as `term_0 ** (1 - spectral_index)`?