Add gas heater to heat source

[DorinevanderVlies]

In the real world of heat networks a gas heater is often used as a backup for, for example, a geothermal heat source.

Request
Add gas heater as an option to the dispatchable source of the heat source list

Notifying @AlexanderWirtz

[AlexanderWirtz]

The ETM uses back-up heaters for the central heat network, but not for local heat networks. This is why they are not available in the ETM SA (according to @ChaelKruip). They should be, though.

In practice, no heat network operates with a single heat source. This could mean people are left in the cold in case of a malfunction. The cheapest and most practical means of back-up is a large gas-fired heater. This can be started-up on short notice and is not too expensive.

as issue #1568 makes clear. We cannot correctly model the O&M/marginal costs of these backup technologies. using a gas-fired CHP is not a good fall-back option, as this would be financially inefficient (a back-up source rarely runs, so maintaining an electricity connection is expensive).