Metadata (only) to represent r10mm.

Renate please review the metadata for r10mm.

No attempt at updating the code to include the count_occurrences functionality.

Note the deliberate step (in the master metadata table) to move towards 'frequency agnostic' standard names.

The metadata also includes a reference keyword (and value) following comment and response. I will let Klaus decide whether to keep it or remove it (temporarily) pending further conversation.