Add JRC IDEES 2021

[khaeru]

A new release of the JRC IDEES data was announced on 2024-05-23, including data for the year 2021: https://data.jrc.ec.europa.eu/dataset/82322924-506a-4c9a-8532-2bdd30d69bf5

To do:

• Check whether code in .jrc (developed for the previous edition with 2015 data; see the documentation) can be used to read the data files from this new release.
• If the data format has changed, make any updates.
• Document support for both the earlier and current editions.