Temperature levels for district heating

[noracato]

Updates the documentation of heat network to the new way we model it.

[mabijkerk]

Transformation of the district heating network to include three temperature:

• Low temperature (LT)
• Medium temperature (MT)
• High temperature (HT)

The transformation includes:

• Added detail for producers that specifies which temperature level heat they can supply
• Added detail for consumers that specifies which temperature level heat they can consume
• Updated calculation for heat_network that allows multiple temperature levels
• Updated costs attributes for district heating
• Updates to slides, inputs and charts
• All initial data is set to HT, meaning that all existing scenarios are migrated to fully take place on the HT district heating network, impact on scenarios is therefore minimal

[mabijkerk]

Atlas: https://github.com/quintel/atlas/pull/160 Documentation: https://github.com/quintel/documentation/pull/163 ETEngine: https://github.com/quintel/etengine/pull/1367 ETSource: https://github.com/quintel/etsource/pull/2952 ETModel: https://github.com/quintel/etmodel/pull/4155 ETDataset: https://github.com/quintel/etdataset/pull/982