Unify data generation into a single function

[kangalio]

Currently each function in data_generators.py iterates over the raw xml file on its own. Datetimes are converted over and over again, no caching. It's probably a bit inefficient, and I think it might also be cleaner to do all the xml analysis in a single place and provide the results in a neat XmlAnalysis class or something. Like it's currently done with replays analysis basically