Fix power connectors again

[pierre-eliep-met]

Goal

Fixes #99 again, update of #278.

What was the problem ?

Power connectors were not balanced because :

• in modelica connectors must always have the same number of flow and potential variables.
• we had only one flow variable in power connectors : W

Solution

• Add a dummy potential variable in connectors
• Set this new variable only once in a power connection set (since potential are equal in every node of the connection set, just like pressure)

Implementation

• A dummy variable is created in power inlet and outlet connectors
• Then, the dummy variable is set in generator inlet power connector (because we always have only 1 generator in a connection set), and same in power sensor inlet and sink inlet
• In all other components with power connectors, a few modifications are made :
  • Gas turbine, compressor : dummy variable of power connectors is set as a connector input
  • pump : no more power connector in pumps, no more need to put a power source along with pumps
  • Steam generator : SGs now have a power connector, and we need to define the dummy of this connector, so I just added a power sink in the SG, and since dummy=0 is defined in the sink, it defines it into the connection set there.

Warnings

Note that this is a bit weird and not super flexible/robust because you could be willing to connect some power components that would both try to set the dummy variable, resulting in an over constrained model.
But it shouldn't be a problem (as of now), because we should never face this issue, because we would probably never have a single connection set with several generators/power sensors/power sinks, etc.

Type of change

• Bugfix
• New feature
• Refactoring change
• Release & Version Update (don't forget to change the version number in package.mo)

Will it break anything in previous models ?

• Breaking change (If yes, make sure to point it out in the changelog)
• Non-Breaking change

Checklist

• I have added the appropriate tags, reviewers, projects (and detailed the size and priority of my PR) and linked issues to this PR
• I have performed a self-review of my own code
• I have checked that all existing tests pass.
• I have added/updated tests that prove my development works and does not break anything.
• I have made corresponding changes or additions to the documentation (in Notion documentation)
• I have added corresponding entries to the Changelog
• I have checked for conflicts with target branch, and merged/rebased in consequence

You can also fill these out after creating the PR, but make sure to check them all before submitting your PR for review.

[pierre-eliep-met]

I've just spotted a problem on compressor unit tests 😭

[valentind-met]

Seems like it's working. However, it seems like a lot of work to be able to implement multi nodes with power connectors. It brings added complexity in understanding the library, with a lot of unused variables...
I am now thinking it would be a more reasonnable solution to use RealConnectors and a "sum" component.