Power to heat: Input, curves, merit order subtype

[louispt1]

This PR implements multiple changes to enable the new Power to Heat temperature setpoint behaviour in ETSource.

There are three main changes in the PR:

1. Four new availability user curves are added to the configuration, as part of the display group power_to_heat_availability

• These curves will set the availability of the P2H technologies

Closes #3329

2. For the following nodes, merit_order.subtype is set to temperature_based_p2h and a default temperature_cutoff of 15.0 has been set on each.

• energy_heat_flexibility_p2h_boiler_ht_electricity
• energy_heat_flexibility_p2h_boiler_mt_electricity
• energy_heat_flexibility_p2h_heatpump_ht_electricity
• energy_heat_flexibility_p2h_heatpump_mt_electricity

The temperature_based_p2h merit_order.subtype is yet to be implemented, but will handle the thermostat-like temperature setting behaviour desired as part of these changes. Under the changes, the temperature_cutoff will determine P2H behaviour by setting the availability of P2H technologies to 0% when the outside temperature is above the setpoint, and setting the availability to 100% when the outside temperature is below the setpoint.

Closes #3328
Related to this Atlas issue

3. The temperature_cutoff_of_energy_flexibility_p2h input has been added, which represents the temperature setpoint slider in the front end. This slider controls the temperature_cutoff attribute of the nodes mentioned in point 2, and takes its gql start value from energy_heat_flexibility_p2h_heatpump_ht_electricity.

Closes #3322

This PR (with this ETModel PR) also Closes #3323

[louispt1]

Note:
When you use ATTACHED_CURVE(curve_name), the model looks for a curve with the given name. For the relevant p2h nodes this refers to the curves dynamically generated in the adapter.