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
There's a few tests on `travis-ci` like this one that fail because you didn't declare to skip them if scipy is not present:
https://travis-ci.org/gammapy/gammapy/jobs/30936680#L1636