Hybrid oil heat pumps in ETM

[KoenvanB]

This PR adds oil-based hybrid heat pumps to the front end. It includes the following additions:

1. Oil HHP slider for households and for buildings;
2. Threshold COP slider for oil HHP

The PR also includes the following changes to existing material:

• Updates of the Demand response - behavior of hybrid heat pumps slide text to refer to general HHPs (instead of only gas-based ones)
• Update of threshold COP slider texts for space heating & for hot water with additional explanation;
• Hides the hybrid_heat_pump table for electricity & gas shares (see corresponding new issue)
• label update for HHP gas & electricity consumer prices (closes #4219)

[mabijkerk]

For e.g., Germany or Northern Ireland, setting space heating 100% to hybrid (oil) heat pumps does not actually turn on the 'hybrid' part. By default, no ambient heat is consumed. The reason for this is that the threshold COP is rather high and decreasing it does lead to ambient heat consumption. I see you have also included this effect in the slider description.

The functionaliteit is therefore verified, but in terms of validation I do wonder whether this is expected behaviour @KoenvanB? This effectively means that under current (or 2019) price conditions and temperatures, the hybrid heat pump will functions as an oil heater. Have you checked this with the client?

[mabijkerk]

Hides the hybrid_heat_pump table for electricity & gas shares (see https://github.com/quintel/etmodel/issues/4237)

This table is still available through the slide text at Demand response - behavior of hybrid heat pumps":

Just like the slider descriptions, the slide text is very text-heavy. If possible I would reduce the amount of text and refer to the documentation when possible.

[KoenvanB]

I have

• shortened the slide & slider texts by including part of it in the documentation (and inserted a reference to the documentation);
• removed the link to the hidden table;
• moved the oil HHP slider position to sit below the hydrogen HHP slider

[KoenvanB]

With the alterations mentioned adjustments this branch is ready to merge!