Renaming (or aliasing?) `online_analysis_interval`

[ijpulidos]

Note that we may also want to consider opening an issue for renaming self.online_analysis_interval to self.offline_analysis_interval

Originally posted by @zhang-ivy in https://github.com/choderalab/openmmtools/issues/592#issuecomment-1149053115

[ijpulidos]

This could potentially break backwards compatibility, even though we have been using the online_analysis_interval for running the _offline_analysis method for a long time now. We could try just making an alias it so that these two point to the same thing, at least for some time. But I guess that when we fix the now experimental _online_analysis method, we want them to be independent.

Another approach could be just using the checkpoint interval for the offline analysis.