add unit commitment for links

[fneum]

Closes #552.

Changes proposed in this Pull Request

Implements unit commitment constraints for Link component in non-pyomo interfaces.

For native, a little generalisation of the UC variable/constraint definition was necessary

For linopy, actually no modifications are necessary except for adding the corresponding attributes to the Link component.

• discussion: start-up and shut-down cost refer to bus0, so one cannot simply copy attributes from Generator to Link for equivalent results, but I think that makes sense.
• [ ] Linopf.py does not have start_up_cost and shut_down_cost when calculating UC problem. #438 should be tackled for this PR to make sense

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

Patch coverage: 91.30% and project coverage change: -0.09 ⚠️

Comparison is base (1790ecf) 76.54% compared to head (0900d14) 76.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #582      +/-   ##
==========================================
- Coverage   76.54%   76.45%   -0.09%     
==========================================
  Files          24       24              
  Lines        6258     6257       -1     
  Branches     1336     1337       +1     
==========================================
- Hits         4790     4784       -6     
- Misses       1186     1188       +2     
- Partials      282      285       +3     

Impacted Files	Coverage Δ
pypsa/optimization/optimize.py	87.79% <ø> (-0.40%)	⬇️
pypsa/linopf.py	80.20% <91.30%> (-0.16%)	⬇️

... and 3 files with indirect coverage changes

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