[Proposal]: Don't output duplicate data in additional files

[cfe316]

Feature description

write_charging_cost outputs ChargingCost.csv.
write_energy_revenue outputs EnergyRevenue.csv.
write_subsidy_revenue outputs [Reg]SubsidyRevenue.csv.
write_operating_reserve_price_revenue outputs OperatingReserveRevenue.csv and OperatingRegulationRevenue.csv.

Each of these have a single column of interesting data, with one row per resource.

All of them are straightforwardly aggregated into NetRevenue.csv.

Motivation

Not outputting these 4-to-6 additional files would remove clutter in the results directory.

Implementation strategy

Remove the csv-writing calls from each. Just return the dataframes, which are already done.

Change the names of these functions from write_* to prepare_* (or some other prefix, like prepare_output_*).

Clean up unneeded columns (R_ID, cluster, zone) from the individual dataframes.

Additional context

No response