Dynamic fuel switching #121

[urwahah]

For #121:

• Added a new fuel switching input "Use optimal heating fuel" under the backup heating equipment selection
• Added new scenarios and scenario group to compare fuel switching
• Added new columns to the loads_to_site dataframe:
  • FUEL_SWITCHING, HHW_REM_W_NG_MODE, BOILER_HHW_W_NG_MODE, GAS_BOILER_WH_NG_MODE, intermediate columns to store the "natural gas mode" load/energy use that it reverts to if gas emissions < electric emissions
  • BOILER_EQ_CALC, BOILER_CAP_W to store the scenario's boiler sizing information as it's used in the emissions function
• Added fuel switching calculation in site_to_source
• Moved the boiler sizing calculation to site_to_source

Here is a link to the change request to update docs for this.

A few minor changes unrelated to the fuel switching:

• Updated the AWHP sizing logic to use the remaining HHW/CHW loads instead of the original load profile. For heat recovery scenarios this will allow the AWHP to be downsized. (This is the change causing the merge conflicts, I think all the new changes should be accepted but wasn't sure if me doing that would also merge the PR)
• Added a 60% efficiency boiler

[urwahah]

i don't love this logic of dropping columns afterward (just because i think it's unnecessary info to output), it might be nicer to call the finalize_columns function here instead of in loads_to_site. but this would mean moving the detail parameter and some small restructuring. i actually think this would be good regardless because a lot of the columns needed in site_to_source are not in the base columns defined in finalize_columns so the calculation would break if detail is ever set to False. also it's just a little annoying to have to list every column passed between loads_to_site and site_to_source.. let me know what you think!