Changes needed for dirac-rucio integration

• Allow slashes in filenames
• Implement lfn2pfn in a way compatible with DIRAC
• Implement extract scope in a way compatible with DIRAC

LFN scheme

This now uses LFNs of the form:

/<VO Name>/<scope>/<path>

Where path can again contain slashes to create a hierarchy inside a VO / scope.

The schema regexp has been changed to allow / in names.

extract_scope has a special case for /<VO Name> that returns the special scope root.

LFN2PFN

Uses the identity function, LFN = PFN