Incorrect parsing of time domain in long format

[danielhuppmann]

As noticed by @dc-almeida in IAMconsortium/nomenclature#487 (comment), casting a dataframe in long format with inconsistent time-domain vs. column name is not automatically corrected.

Expected behavior:

• if all values in a 'time' column are integer, the column should be renamed to 'year'
• if (at least) some values in a 'year' column are datetime-castable, the column should be renamed to 'time'