Recommend assert_allclose instead of assert_almost_equal

[IAlibay]

See https://github.com/MDAnalysis/mdanalysis/pull/3425#discussion_r715923852

We probably should change our base recommendation in the userguide to use assert_allclose?

[tylerjereddy]

Not sure if it will be deprecated anytime soon--I imagine it is used in thousands/millions of lines of test code, but maybe doesn't hurt to be consistent with latest recommendation yeah.

I don't know if I'd advocate for mass replacement yet, but perhaps just a suggestion for new code