Refactor to `nomenclature-iamc` package

This release follows the refactoring of this repository to use a new dedicated package nomenclature-iamc (see https://github.com/iamconsortium/nomenclature) instead of an ad-hoc solution implemented for this project.

First installable nomenclature package

This is the first release of the installable Python package to work with the openENTRANCE nomenclature.

Disclaimer: there are several known inconsistencies in the definitions, so please be aware that many terms may still change!