Aggregation issue for seasonal indices and cell methods etc.

Metadata for seasonal indices appears to be spread out in several issues. This issue is just to collect these into one:

Focus on cell methods produced by climix

#52 Correctly implement cell_methods
#193 Non-CF cell methods should be empty
#246 cell_methods for seasonal indices

Focus on clix-meta --> climix interface

#95 annual indices based on part of the year are not seasonal
#170 Mechanism to specify user defined seasons in the master table

Focus on index definition in clix-meta

clix-meta #29 Clarify description of period