Reorganize emissions info #121

[urwahah]

Fuel switching (and maybe load shifting later on) requires the grid emissions data to be available for loads to site energy calculations. This is a fairly significant revision to the architecture of energy.py so it would be better to review and implement this before I start changing the calculations :-)

To summarize the changes:
Currently:

1. run_loads_to_site calls loads_to_site_energy, which iterates through the equipment scenarios, and saves the site energy results as a temporary file
2. run_site_to_source passes those results to site_to_source, which iterates through the emissions scenarios for final source emissions

Proposed:

1. run_loads_to_site calls loads_to_site_energy
   a. loads_to_site_energy calls grid_emissions, which iterates through the emissions scenarios to return a dataframe containing grid emissions data
   b. loads_to_site_energy iterates over both emissions and equipment scenarios (nested) to calculate site energy
2. run_site_to_source passes the site energy results file (same as before) to site_to_source, which iterates through the emissions scenarios for final source emissions

[urwahah]

unrelated but i changed this because it was doing a cumulative sum of all emissions scenarios which didn't seem helpful to me. now it outputs the total emissions for each scenario - still summed up for all equipment scenarios, but better for a high level check i suppose