Add marginal emissions estimate outputs

[grgmiller]

There is interest among some stakeholders in marginal emissions estimates being included in the OGE results. There are many different methods for estimating marginal emissions, so one value add that adding MEFs to OGE could provide could be to provide MEF estimates using several different methods so that they can be compared side-by-side.

Different methods that we could potentially implement using the data in OGE include:

• Statistical dispatch-based estimates
• Statistical emissions-based estimates
• Non-baseload estimates (https://github.com/singularity-energy/open-grid-emissions/issues/76)

Merit-order and LMP-based estimates would not be relevant since the dataset does not currently include price information.