obs_seq header synonyms

[hkershaw-brown]

The 'observation' line in the obs_seq header can have several (any? user defined) values

e.g

synonymous_copy_list = 'NCEP BUFR observation', 'AIRS observation', 'observation'
synonymous_copy_list = 'GTSPP observation', 'SST observation', 'observation'

https://github.com/NCAR/DART/blob/27f85ee4794c4ffb18ca84eef1ffc2265dae333d/assimilation_code/programs/obs_sequence_tool/obs_sequence_tool.rst?plain=1#L457

https://github.com/search?q=repo%3ANCAR%2FDART%20synonymous_copy_list&type=code

The init is expecting an 'observation' column to be present when calculating bias, sq_err

[hkershaw-brown]

there are a bunch of synonyms, WOD observation

I think this will have to be user supplied (in addition to a list of possible dart ones).