Coupling generators

[FabianHofmann]

Minimal invasive alternative to #549
closes #549

From release notes:
It is now possible to couple the dispatch of generators. For this purpose, three new columns p_coupling, p_coupling_coeff and p_coupling_sign have been added to the generator component. They allow coupling the dispatch of one generator to another generator by adding a linear constraint to the optimization problem. The condition is of the form p {p_coupling_sign} {p_coupling_sign} * p_{p_coupling}. The coupling is applied only when the coupling generator is active.

See the example here: https://pypsa--738.org.readthedocs.build/en/738/examples/generator_coupling.html

This change is purely additive and does not remove any functionality. It might be quite helpful for pypsa-eur as it allows keeping the generators coming from "electricity only" and complement them via additional coupled input/output generators.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[codecov]

Codecov Report

Attention: Patch coverage is 85.18519% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 66.31%. Comparing base (baa242b) to head (8687a8b).

Files	Patch %	Lines
pypsa/components.py	50.00%	1 Missing and 1 partial ⚠️
pypsa/optimization/constraints.py	90.90%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #738      +/-   ##
==========================================
+ Coverage   66.24%   66.31%   +0.06%     
==========================================
  Files          26       26              
  Lines        7063     7089      +26     
  Branches     1418     1423       +5     
==========================================
+ Hits         4679     4701      +22     
- Misses       2102     2104       +2     
- Partials      282      284       +2     

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