Initial migration to Quantities

The `assert_approx_equal` docstring has a note recommending the use of other `assert` functions:
http://docs.scipy.org/doc/numpy/reference/generated/numpy.testing.assert_approx_equal.html

In Gammapy I exclusively use `assert_allclose` now because it's simple to use and covers all use cases ... and as far as I see `assert_allclose` is also being used more and more in Astropy.