Modify code to use nomenclature from IAMC

[gxexakis]

Substitute the validation methods with the ones from nomenclature for consistency with the community.

Modify the checking code so that it returns results as a pyam.IAMDataFrame or pandas.DataFrame, which users can then process themselves if they want. Then use that to create the output files, as a formatted Excel file as before, but optionally as a CSV or other less slow format for large datasets.

proposed by Jan Ivar Korsbakken