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 (closed) Non-CF cell methods should be empty
• #246 (closed) cell_methods for seasonal indices

Focus on clix-meta --> climix interface

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

Focus on index definition in clix-meta

• clix-meta #29 Clarify description of period