Uniform analysis results with MDA

[RMeli]

Expected behaviour

I expect to access the results of PMDA in the same way I would do with MDA.

Actual behaviour

After https://github.com/MDAnalysis/mdanalysis/issues/3115 the results in MDA are stored in a self.results, therefore this has diverged from PMDA.

[orbeckst]

PMDA needs a big lift to bring it in-line with MDA 2.0.