update hole2 analysis to use .results

[orbeckst]

Follow the new AnalysisBase API and store results in HoleAnalysis in .results instead of class attributes.

This should be done for 2.0.0 #3254 so that we don't have to start this new analysis module with deprecations in place.

See omnibus issue #3261 for background and details. See PR #3263 as an example (DensityAnalysis) but note that no deprecations are necessary — just change where the results are stored and update the docs.

• use attributes inself.results instead of self
• update docs
• add an entry in CHANGELOG under Changes (for completeness)

[lilyminium]

I can do this after #3281 is done.

[orbeckst]

@lilyminium I approved PR #3281 — I don't know if @PicoCentauri has anything to add there. But if you're pressed for time I suggest you merge the PR whenever you want to finish this one.