Add external coupling for input and output conversion of liquid fuels

[robindesmit-quintel]

Context

We want to introduce an external model to adjust the conversion ratios of the liquid fuels processes. The ETM should include an option to couple this external model to the original ETM.

Implemented changes

• Add the liquid fuels extension as an external model to the coupling group so that it can be activated and deactivated. (coupling_group_wrapper)
• Add the external coupling liquid fuels sliders. (external_coupling_efficiency_fuel_production_process)
• Add names to the external coupling liquid fuels sliders. (en/nl_external_coupling)
• Add names to headings above the sliders. (en/nl_etm)
• Add all liquid fuels technologies as subheadings to "Liquid fuels and feedstock". (cost_fuel_production.yml)
• Add name and description of subheadings. (en/nl_costs)
• Adapt the costs sliders location and names to fit the new layout.

Related

Goes with pull requests #3452 (quintel/etsource#3452)

Checklist

• I have tested these changes
• I have updated documentation as needed
• I have tagged the relevant people for review

[mabijkerk]

@robindesmit-quintel I would sort the sliders a bit more, so that the inputs are first and bundled together, then the outputs bundled together. Now they are all mixed: