Create importers for core data sources

[clizbe]

Despite their formats changing year to year, this is something we should build up. At least with importers for current datasets, we (TNO) can learn how to create them for future formats as well.

Possible sources:

• [ ] #30
• [ ] ENTSO-E -> Can't access the database, just the published in Excel files
• [ ] ENTSO-G -> ditto
• [ ] CorRES
• [ ] KEV(?)
• [ ] II3050
• [ ] IO2050
• [ ] IP2024
• [ ] TYNDP 2022
• [ ] AIDRES
• [ ] (Future) Look into OPERA sources
• If I'm not mistaken, this is PyPSA's data: https://github.com/PyPSA/technology-data/tree/master/outputs

[clizbe]

This is also a suggested task for the Micro-Learning Community.