General enhancements for standardization

[knoiva-indecon]

General enhancements for standardization and efficiency (will require coordination with FrEDI Data), in order of staging:

1. ImpactYear label. Change "NA" label in impactYear column of run_fredi outputs to "N/A" instead of "NA" (use label instead of id) for consistency. All results will be different if impactYear is not included in aggLevels argument.
2. Scenarios. Add steps to "Build FrEDI" workflow to update scenarios in "data" and "inst/extdata/scenario" directories of FrEDI package. Also include the GDP scenario (see Issue Default scenarios #85)
3. Remove "byState" arguments and steps throughout run_fredi and helper functions. Can still use popCols = c("state", "postal"). Change the name of the population column from state_pop to just pop.

May add additional standardizations to come.

[knoiva-indecon]

This issue is addressed in the fredi_methane_reorg branch, and will be merged into main with pull request #201. It can be closed after merging into main.