Dynamic fuel switching #121 - #154

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