enable EQ constraint on locations

[martacki]

Closes # (if applicable).

Changes proposed in this Pull Request

enable the EQ constraint to operate on locations, particularly useful for sector coupled models

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.

[tgi-climact]

Hi @martacki , I am also interested by this feature. If you intend to make it work for sector-coupled models, it seems to me that we should also add links related to power supply into the constraint. Am I missing something ?

[martacki]

Hi @tgi-climact thanks for your interest in the feature and remark! You're absolutely right. Needs some filtering though, I think, so that we always take the right pX stream (i.e. p1, p2, ...), if applicable. Need to have a close look into the generators, too.

I didn't have any generating units in my network that were attached by links, so I'll need to find a new test-case for this

[koen-vg]

Related to #659? Unfortunately I believe implementing the EQ constraint for sector-coupled networks can be a bit complicated (see the docstring of the rewritten EQ constraint function in the aforementioned PR).

(P.S. I do hope to find the time to revisit #659 and see if I can finally get it merged. Looking back at it now it does seem like maybe it's more complicated than it needs to be so I'll be on the look-out for simplifications.)