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
This doesn't belong in `gammapy.catalog` ... should go somewhere in `gammapy.utils` ... maybe `gammapy.utils.array` is better than starting a new sub-module?
(This should go away anyways with Astropy 1.0 when Table / Quantity integration happens (hopefully)).