68910816 959 refactor data point output attributes

Attribute now has a source. OSRunner and RunManager set the source. Source gets serialized to xml, json, and OSP.

DataPoint now pulls output attributes from xml and ossr. DataPoint json and osp no longer tries to keep track of its xml (or ossr) files, but instead just saves its outputAttributes directly. Xml and ossr file references lazy load as requested. (That is, the API of DataPoint did not change.)
@elainethale It seems like this used to be doing some caching but is no longer doing that, is that intended?  I am seeing significant performance issues on this branch and wondering if this is the cause?