Average emissions factors #52

[urwahah]

For #52:

• Added two new inputs:
  • Electricity emissions source, with the following options: Average (Cambium), Constant (User-provided), Marginal (Cambium, Long-run), Marginal (Cambium, Short-run). Timeseries (User-provided) to be added in the future to address User uploaded emissions factors #57
  • Constant grid emissions rate, no default
• Added calculations for constant emissions and Cambium average emissions (using the AER load metric)
• Revised marginal emissions calculation to automatically assign short-run weighting based on the emissions source selected
  • Removed time zone and short-run weighting attributes/inputs from the emissions data/scenarios
• Added a new emissions scenario group to compare all four options
• Added emissions scenario name to the output table

[t-kramer]

@urwahah Great work, thanks! I merged this and will implement a few minor fixes myself.