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.
I used all kinds of asserts in the past, but these days I pretty much only use `np.testing.assert_allclose`, because I find that it's simple to use and covers all use cases. But obviously that's a matter of taste ... I think `pytest` is super-smart and prints the values of `fluxerr`, `true_error` and `0.01` here if the test fails?