package organization with future updates in emission factor data sets

[rafapereirabr]

Emission factor models are regularly updated, often with new emission factors data sets every year.

We need to think carefully how will deal with these updates. Here are three questions that immediately come to mind.

1. Will gtfs2emis incoprate new data sets internally? or should we create a separate data package to store emission factor models that gtfs2emis would ingest?
2. What parameter name should we expose to users so they can choose the year/edition of emission factor models?
3. How would we deal with deterioration of fleet in a way that is transparent to users?