Customize output metadata

[JamesAnstey]

In get_variables_metadata:

• -a option to specify metadata attributes to include in the output
• `-r' option to filter variables based on realm
• clarified either CMIP7 or CMIP6 compound names can be used to filter variables (-cn option)
• clarified -t option refers to CMIP6 (not CMIP7) CMOR tables
• make csv output columns exactly same as fields included in json output

Intention of -a is that user can easily prune metadata they don't want, to avoid cluttering the output. May be useful for unharmonised request to provide MIPs a starting list of variables with simplified metadata. Example:

get_variables_metadata v1.2.2.3 vars.json -a modeling_realm,frequency,region,long_name,standard_name,units,comment,processing_note,dimensions,cell_methods,cell_measures,positive,type,variableRootDD,branding_label,branded_variable_name

or to produce a csv file instead, replace vars.json with vars.csv.