Adding more tests for aperture_photometry

This PR depends on #68 and can be merge after that one, once I rebase. 

Coverage is 80% at the moment, only the ``mask is not None`` case is missing.

Addresses #64.
Can you rename `BaseTestErrorGain` to something that doesn't have `gain` in the name?

In my opinion being able to process data with `gain` is a special case and shouldn't play such a prominent role as the base class for all tests.