Add `data_source` column to all emission factor tables

[grgmiller]

We should add a "data source" column to all emissions factor tables in data/manual, including:

• emission_factors_for_co2_ch4_n2o
• emission_factors_for_nox
• emission_factors_for_so2
• geothermal_emission_factors

This will allow for greater transparency and also allow us to keep track of where the data is coming from once we start updating these sources (see https://github.com/singularity-energy/open-grid-emissions/issues/69).

The co2 emission factor table already has a source column but we should update this to be more specific with the name of the document and even the number of the table/figure if possible.