API docs for `gauss2d` function not generated because `Gauss2D` class shadows

Now that https://github.com/astropy/astropy/pull/1826 is merged I get an error because in `gammapy.morphology` I have a `gauss2d` function and a `Gauss2D` class:
```
/Users/deil/code/gammapy/docs/morphology/index.rst:30: WARNING: toctree references unknown document u'api/gammapy.morphology.gauss2d'
None:None: WARNING: toctree contains reference to nonexisting document u'api/gammapy.morphology.gauss2d'
<autosummary>:None: WARNING: toctree contains reference to nonexisting document u'api/gammapy.morphology.gauss2d'
```

The ``docs/api/gammapy.morphology.gauss2d.rst`` file is no longer generated:
```
$ ls -1 docs/api/gammapy.morphology.*2*
docs/api/gammapy.morphology.Gauss2D.rst
docs/api/gammapy.morphology.MultiGauss2D.rst
docs/api/gammapy.morphology.ThetaCalculator2D.rst
docs/api/gammapy.morphology.delta2d.rst
docs/api/gammapy.morphology.shell2d.rst
docs/api/gammapy.morphology.sphere2d.rst
```

CC @kbarbary @eteq @astrofrog Can you fix this?