Add validation notebook for 2030 demand prediction

[pz-max]

Add demand validation script for Africa in 2030t Our data is as total in the range of other 2030 predictions. However, a first investigation suggests that other prediction years and continents need to be inspected in detail and adjusted if required. GEGIS did not solve for all countries which is why we used heuristics that are error-prone.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• A note for the release notes doc/release_notes.rst is amended in the format of previous release notes, including reference to the requested PR.