Use the iam-nomenclature package

[znicholls]

Credit to @danielhuppmann for the original wording of this issue.

The definitions of variables could be made to follow the https://github.com/iamconsortium/nomenclature package, which has some utility features to "build up" an IAMC-style variable tree using .

The nomenclature package could be used in this repo for the following:

• provide some utility features to provide a complete list of variables and units to downstream users
• ensure that all definitions-yaml-files PR-ed to this repo are parse-able via nomenclature.testing.assert_valid_yaml() https://github.com/IAMconsortium/nomenclature/blob/437c0cd350747e454ff221566068a30fc1146e4d/nomenclature/testing.py#L9
• ensure that any IAMC-style data generated from the workflows is in line with the definitions (including correct units)

The nomenclature package will also have a feature to check for valid ranges, see IAMconsortium/nomenclature#19