Storing paths to external files

[catherinebirney]

@matthewlchambers @bl-young

I took a first attempt at implementing a system to load files stored externally from FLOWSA. Paths to the files are stored in "external_paths.env", similar to "API_Keys.env". An example is here.

Right now these file paths are not implemented as a means to load an externally saved FBA, we are using the external filepaths in WARM-USEEIOdocs to load csv files. See 'designated_mapping_file' and 'append_material_codes' here.

The current implementation will need to be modified for loading non-csv files.