update helix_analysis to use .results

[orbeckst]

Follow the new AnalysisBase API for the new helix_analysis and store results 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)

[orbeckst]

@fiona-naughton @lilyminium (or anybody else) helping hands would be much appreciated!!

(See #3261 (comment) ...)

[orbeckst]

When you look at this issue please also fix #3267 .

[lilyminium]

is there a deadline for this :)

[IAlibay]

is there a deadline for this :)

gotta be merged before we release 2.0.0, Friday ;) [mayyybeee Saturday, but really Friday]