Method description should be more flexible

[bl-young]

e.g., information about mapping is not needed for inventory methods:

https://github.com/USEPA/LCIAformatter/blob/b16f092a4ec4f23b56dabe6bfac86fa8a55fee71/lciafmt/data/description.yaml#L10-L20

[bl-young]

removed mapping portion when not applicable with ce98823