`VTXTemperatureExport` isn't supported with `HydrogenTransportProblemDiscontinuous`

[RemDelaporteMathurin]

Right now the temperature export is not implemented with HydrogenTransportProblemDiscontinuous.

FESTIM/src/festim/hydrogen_transport_problem.py

Lines 1707 to 1710 in 1428092

	if not isinstance(export, exports.VTXSpeciesExport):
	raise NotImplementedError(
	f"Export type {type(export)} not implemented"
	)

This should be fairly easy though as we don't need to split the temperature in subdomains but simply export the parent temperature

Thanks @tez-orr123 for reminding us