Must-run interconnectors curves functionalities should be fixed

[mabijkerk]

Issue
In this scenario, I have setup must-run interconnectors. With the following active couplings and inputs:

external_model_interconnectors
interconnector_2_must_run

external_coupling_interconnector_2_must_run: 1.0
electricity_interconnector_2_capacity: 1000.0

I have also uploaded a flat profile with the value 0.5 for both import and export. According to our front-end, these profiles should read as capacity profiles. This means I would expect the FLH of both import and export to be 8760*0.5 = 4380 and therefore 4380000 MWh (or ~ 16 PJ) flows. However, when I look at the electricity balance, there is 0 PJ supply from import and ~ 20 PJ demand from export.

This shows that the must-run interconnectors do not take their FLH from the uploaded profiles, but instead from the node files. The FLH specified in energy_interconnector_2_imported_electricity are 0.0 and in energy_interconnector_2_exported_electricity are 5427.6084, which explains the behaviour in the chart.

Goal
The newly introduced must-run types, export_must_run and import_must_run, take their FLH from the node file. They should take the FLH from the must-run profile, when one is uploaded. Notifying @kaskranenburgQ and @louispt1. Ideally we fix this in January before the stable version is created.

When we implement a fix, we should check the behaviour when must-run is active, but no curve has been uploaded yet.

[mabijkerk]

The issue outlined above is must-have in order for the functionality to work as expected. There are two additional, but smaller issues that we should also investigate if we're updating the functionality anyway.

1. When the coupling is activated, it is no longer possible to upload a price curve. This should still be possible, because deactivating the coupling will then 'enable' the normal flexible behaviour of the interconnector. When the coupling is active, the curve should simply be ignored.
2. Instead of having to upload separate must-run curves, we should ideally implement that the availability curves are taken as the must-run curves when the coupling is activated. This would reduce the amount of data required to provide.

• Must-have: read FLH from curves, instead of from nodes
• Good-to-have: allow uploading of price curve, even when coupling is activated
• Nice-to-have: 'merge' availability and must-run curves