document assert_allclose and pytest.approx

[IAlibay]

Since these are the favoured assertion methods for the core library, we should document these in testing.rst.

[jandom]

This ticket sounds very similar to #171 – what's the difference? pytest.approch?

[IAlibay]

@jandom yeah the addition here is pytest.approx. MDAnalysis recommends using approx for single float comparisons and allclose for arrays (it's purely a speed thing, allclose on single numbers is very slow).