global_constraint: add operational limit constraint

[FabianHofmann]

Analogous to primary_energy_limit but directly working on carrier output. This will help to replace some stores in pypsa-eur (sector-coupled) by light-weight constrained 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.

[fneum]

Can you explain a little more about how it works? I don't yet understand why it is better than a Store.

[codecov]

Codecov Report

Patch coverage: 77.50% and no project coverage change.

Comparison is base (c8d9b7e) 76.64% compared to head (49c472c) 76.65%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #618   +/-   ##
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  Coverage   76.64%   76.65%           
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  Files          24       24           
  Lines        6290     6330   +40     
  Branches     1331     1338    +7     
=======================================
+ Hits         4821     4852   +31     
- Misses       1188     1194    +6     
- Partials      281      284    +3     

Impacted Files	Coverage Δ
pypsa/optimization/global_constraints.py	85.65% <76.92%> (-1.56%)	⬇️
pypsa/optimization/optimize.py	88.18% <100.00%> (+0.04%)	⬆️

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[FabianHofmann]

The reason is that modelling stores is quite extensive. You have intertemporal constraints with auxiliary variables (soc, charge, discharge). If you limit the total energy output of one single generator (or a generator carrier fleet) instead of setting up a store with an initial soc which depletes over time, you save a lot of complexity. So (reducing to generators) the constraint is just saying

$$\sum_{G \in \Theta, t} g_{G, t} <= limit$$

where $\Theta$ is the set of generators which belong to the carrier set in the global constraint.

[fneum]

Ah, I see that makes sense.

[fneum]

Looks good! Nicely tested, so it was easy to review.

[fneum]

At some point, we need to update the documentation of the GlobalConstraint component:

https://pypsa.readthedocs.io/en/latest/components.html#global-constraints

[tintin305]

I don't see any place that caters for selecting investment_periods. Will this still need to be introduced as a parameter?