Update to include CMIP3 mapping

[durack1]

The CMIP3 data needs to be mapped to the CMIP6 specs so that CMIP3, 5 and 6 can all be addressed using the same directory and filename logic. To achieve this, the CMIP3 tables that were used for CMOR1 will need to be uncovered so a mapping can occur.

I'll obtain a copy of these tables from @taylor13 (which are summarized at https://pcmdi.llnl.gov/ipcc/standard_output.html), update them into an appropriately named github repo and use these as input for the mapping

[durack1]

Relevant to this task is cmip3-cmor-tables and the data stored on CSS03 at scratch-cmip3

[durack1]

Edits required
lookupCMIPMetadata https://github.com/pochedls/xagg/blob/master/fx.py#L100-L116
produceCMIP5Activity https://github.com/pochedls/xagg/blob/master/fx.py#L120-L154
createGridLabel https://github.com/pochedls/xagg/blob/master/fx.py#L169-L261
parsePath https://github.com/pochedls/xagg/blob/master/fx.py#L264-L377

[pochedls]

@durack1 - I added the gridLabel documentation to the fx.py file.