Add AP-42 emission factors

Available from [US EPA](https://www.epa.gov/air-emissions-factors-and-quantification/ap-42-compilation-air-emissions-factors). In PDF form; possible that there are electronic versions available.

Start with the US EPA AP-42 source linked in the issue and inspect how this repository populates its relational database. Determine whether an electronic source is available and which data format the project accepts; done means the AP-42 emission factors are added to the database in the expected form.

Written by the indexing model from the issue text.