Add some more TODO items
Fix PcpArchive.get_values class description
Use pmGetArchiveLabel() to get the start of the archive

Avoid doing a pmFetch() just to get the start of the archive
Clean up the parsing of metrics

Use the suggestions from Nathan Scott on the pcp mailing list
in order to rewrite the PcpArchive.get_values() method. Now
we scan the archive only once and fetch all the metrics through
the use of pmFetchArchive.
Store more info about each metric in the pmns dictionary