metadata API endpoint should include climatological information

[rod-glover]

The metadata API endpoint should be extended with the following properties:

• multi_year_mean: from TimeSet.multi_year_mean
• start_date, end_date: from TimeSet.start_date, TimeSet.end_date, which now contain climo bounds for multi-year mean files (see Store climo bounds in TimeSet.start_date, end_date for mym files modelmeta#12)
• timescale: from TimeSet.timescale (see also issue Indexer sets TimeSets.time_resolution incorrectly for some climo datasets modelmeta#9)

All values derived from TimeSet are None if there is no such record associated to the DataFileVariable for the requested file.