[thredds] CF Discrete Sampling Geometries conventions - final(ly)

We finally finished the CF Discrete Sampling Geometries conventions - this is now an accepted part of CF conventions version 1.6.

https://cf-pcmdi.llnl.gov/trac/attachment/ticket/37/CFch9-may4.docx?format=raw

There were some changes in syntax - the CDM will be updated to reflect these. If you have concerns about backwards compatibility, please contact me.