feature/validation_integration

[sagemaso]

Fixes openego/eGon-validation#10 and openego/eGon-validation#11

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[nesnoj]

@sagemaso please make sure you follow the checklists above before converting to "ready for review".

On the failing tox build: as mentioned it is related to #1386 and will be fixed by #1382 but this can take some time.
So I temporarily limited setuptools to <81 in d53a04c and 8bc839c and tox runs successfully.
Before the merge, these commit needs to be reverted!