fix sector coupeled model for isolated non EU28 countries, e.g. UA

[martacki]

contents: Just making the sec model run by filling missing sectoral data with defaults, average EU values or zeros, if nothing else is available. Thus, it currently has no real sectoral data, but allows the execution of the power model only in the sector coupeled style (i.e. adding distribution grids, solar thermal, etc.)

other comments: experimenting to use git cherry-pick from OET/pypsa-eur fork to align our master branch with upstream ... (was this PR in OET fork https://github.com/open-energy-transition/pypsa-eur/pull/1)... unfortunately used the wrong git commands, so that the history was lost

Checklist

• [x] I tested my contribution locally and it seems to work fine.
• [x] Code and workflow changes are sufficiently documented.
• [x] Changed dependencies are added to envs/environment.yaml.
• [x] Changes in configuration options are added in all of config.default.yaml.
• [x] Changes in configuration options are also documented in doc/configtables/*.csv.
• [x] A release note doc/release_notes.rst is added.