Harmonize upstream inventory processing

[dt-woods]

Consider harmonizing both the methods and the code for reading and formatting the upstream inventory data.

A secondary consideration is deciding whether these inventory formatting methods could be moved into their own package. Particularly, when considering #272. They should be agnostic to the eLCI modelconfig parameters (e.g., a pre-processing step that reads, formats, and prepares upstream inventory data to be merged with generation data).