Rename gammapy.data to gammapy.datasets

I suggest to rename `gammapy.data` to the more explicit `gammapy.datasets`.

It might also be a good idea to start all function names with `load_`, `fetch_` or `make_` like `sklear.datasets`.

* [sklearn.datasets](http://scikit-learn.org/stable/modules/classes.html#module-sklearn.datasets)
* [skimage.data](http://scikit-image.org/docs/0.9.x/api/skimage.data.html)