Changes proposed in this Pull Request

Make sure PyPSA features fully functional modular expansion with stochastic networks.
Add related tests.
Also handle it within consistency_check (set as scenario invariant).

Also closes #1497

Comment

I leave a few line comments wrt possible use of a new property helper c.modulars (this would make sure code for nominal and modular variables/constraints is similar). That is, however, not strictly necessary: there's little code needed for this PR, so it is likely that gains do not justify setting new property helper.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in docs.
• Unit tests for new features were added (if applicable).
• A note for the release notes docs/release-notes.md of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.