Simplify some energy_source & fuel_type values.
Simplify some energy_source & fuel_type values.

On ingest, create simplified versions of:
 * boiler_fuel_eia923.fuel_type
 * fuel_receipts_costs_eia923.energy_source
 * fuel_receipts_costs_eia923.fuel_group

Probably this type of operation should be moved into the clean_eia923
module for consistency, across all of the ingest functions...

catalyst-cooperative/pudl,catalyst-cooperative/pudl