update polymer.PersistenceLength to use .results

[orbeckst]

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

See omnibus issue #3261 for background and details. See PR #3263 as an example (DensityAnalysis)

• put all generated data into self.results instead of self
• keep managed attributes in place with deprecation message
• test deprecation messages/old attribute access
• update CHANGELOG: one entry in Changes (use .results) and one entry in Deprecations (attribute access to be removed in 3.0.0)