Remove sub-module hierarchy for gammapy.spectrum

All functionality in `gammapy.spectrum` should be exposed in that namespace (except maybe `gammapy.spectrum.models` if there are too many).
I.e. users should not have to import from sub-modules like `gammapy.spectrum.crab`.

The number of user-facing functions / classes should be reduced and names should be made more explicit where needed so that they make sense in `gammapy.spectrum`.