Improve process to update OTH source code

[rouille]

Some generators have a reported energy_source_code of OTH. Currently, we manually replace OTH with another fuel code that best matches the fuel actually burned at the generator. This is done in notebooks/manual_data/manually_update_OTH_fuel_code.ipynb. Using this notebook, the src/oge/reference_tables/updated_oth_energy_source_codes.csv file is updated.

When a generator has OTH as energy_source_code for several report_date, it is possible that more than one energy_source_code. Looking at plant 10673 for years 2010 - 2014, there are four months with have OTH as energy_source_code:

        plant_id_eia report_date energy_source_code  fuel_mmbtu_per_unit  fuel_consumed_mmbtu
64405          10673  2010-07-01                OTH                24.41               8324.0
84034          10673  2010-09-01                OTH                24.41              49430.0
93852          10673  2010-10-01                OTH                 5.67              17843.0
103681         10673  2010-11-01                OTH                 5.67               6186.0

One can see that the fuel_mmbtu_per_unit can take two values that point to 2 different fuel. I f we look at the data for this plant for other months:

        plant_id_eia report_date energy_source_code  fuel_mmbtu_per_unit  fuel_consumed_mmbtu
5614           10673  2010-01-01                BIT            22.549999            1394199.0
5619           10673  2010-01-01                 WO             5.670000               3215.0
15401          10673  2010-02-01                BIT            22.230000            1172744.0
15406          10673  2010-02-01                 WO             5.670000               2688.0
25194          10673  2010-03-01                BIT            21.180000            1255487.0
25195          10673  2010-03-01                DFO             5.670000               2336.0
25199          10673  2010-03-01                 WO             5.670000              10654.0
34994          10673  2010-04-01                BIT            20.879999            1209244.0
34999          10673  2010-04-01                 WO             5.670000               6617.0
44792          10673  2010-05-01                BIT            21.590000            1172056.0
...              ...         ...                ...                  ...                  ...
601019         10673  2014-10-01                DFO             5.670000                 34.0
601022         10673  2014-10-01                TDF            31.000000              12896.0
601023         10673  2014-10-01                 WO             5.670000               5364.0
612446         10673  2014-11-01                BIT            20.656000            1338922.0
612450         10673  2014-11-01                TDF            31.000000               8432.0
612451         10673  2014-11-01                 WO             5.670000               5398.0
623889         10673  2014-12-01                BIT            20.625999            1296922.0
623890         10673  2014-12-01                DFO             5.670000               1956.0
623893         10673  2014-12-01                TDF            31.000000               9796.0
623894         10673  2014-12-01                 WO             5.670000               6594.0

it seems to indicate that OTH can be WO or DFO (5.67) or BIT or TDF ([20,31]).

We probably want to include the report_date in the src/oge/reference_tables/updated_oth_energy_source_codes.csv file and use an algorithm that fills OTH using the energy_source_code reported before/after.