fixedMultiPhaseHeatFlux: Calculates a wall temperature that produces the specified overall wall heat flux
across all the phases in an Eulerian multi-phase simulation.

Intended to be used with copiedFixedValue to ensure that phase wall
temperature are consistent:
    - Set 'fixedMultiPhaseHeatFlux' boundary for one of the phases
    - Use 'copiedFixedValue' for all the other phases.

Based on code provided by Juho Peltola