Implementation of morphology models as astropy models and testing

Implementation of morphology models as astropy models and testing. 
Sphinx links now always should point to where the user is supposed to import from, not the file location.
In this case
```
astropy.modeling.Gaussian2D
```
The Sphinx build currently shows a few warnings that should also be fixed:
```
/private/tmp/gammapy/docs/api/gammapy.morphology.Delta2D.rst:7: WARNING: py:class reference target not found: astropy.modeling.core.Fittable2DModel
/private/tmp/gammapy/docs/api/gammapy.morphology.Shell2D.rst:7: WARNING: py:class reference target not found: astropy.modeling.core.Fittable2DModel
/private/tmp/gammapy/docs/api/gammapy.morphology.Sphere2D.rst:7: WARNING: py:class reference target not found: astropy.modeling.core.Fittable2DModel
```