clean up metadata approach

While #343 improved the link between pyPRMS metadata and pywatershed, the code is still very ad hoc and potentially difficult to to understand. The code could be refactored. Some other approaches could be considered as well. Among them, relying on pyPRMS to provide the metadata instead of parsing its XML files directly. This would require tests and stability on the pyPRMS side for the metadata class.

Start by reviewing the metadata changes from #343 and the current pyPRMS-to-pywatershed integration. Examine whether pyPRMS can provide the metadata instead of direct XML parsing, including the tests and stability needed for its metadata class; done requires an agreed approach and coverage for the refactored metadata handling.

Written by the indexing model from the issue text.