slow loading FBS yaml files

[bl-young]

FBS method files that are heavy with !include are slow to load.

e.g., GHG_national_2020_m2.yaml.

Relatedly, even when just accessing an FBS, the method file must be loaded here, which I think is unnecessary unless the FBS isn't found and needs to be generated.

[bl-young]

This also is slowing the CI/CD test substantially

[bl-young]

adjusted FBS method testing in https://github.com/USEPA/flowsa/commit/ea115eb5b422fafaf5cf107a02b99966ecb51432