Include GENUNITS table from mms

[dec-heim]

Request/update to include GENUNITS table from mms, in particular to allow users to access 'CO2E_EMISSIONS_FACTOR' for a specified date range.

Suggested commits are setup to remove duplicated rows on primary keys: ['GENSETID', 'LASTCHANGED','CO2E_EMISSIONS_FACTOR']. This way the user can also clearly see different assumptions for CO2 factors updated overtime from NTNDP/ISP in 'CO2E_DATA_SOURCE' col

• Inclusion of 'GENUNITS' in defaults.py
• Inclusion of 'GENUNITS' in processing_info_maps.py

[prakaa]

Signing off but some things to consider:

1. The filter function used by this table is untested (though relatively simple). We will need to add tests:

2. Guidance on table use would be good to document. As you suggested, might be best to have this as a wrapper func in `NEMED` @dec-heim. Ideally, links to the various sources are included in docs:

3. Wiki needs to be updated

[dec-heim]

For those interested... this functionality for GENUNITS + DUALLOC is used in and also available via the package NEMED.
See https://github.com/UNSW-CEEM/NEMED/blob/master/src/nemed/downloader.py