BUG: externalWallHeatFluxTemperature: refValue=0 leads to FPE in mixedEnergy BC

In light of Jiri's failing test case:

In externalWallHeatFluxTemperature BC, the modes fixedPower and fixedHeatFlux set refValue to 0.

This upsets the cases where rho is interrogated as a function of p and T values, e.g. for the Foam::perfectGas<Specie>::rho when the sonicFoam is used (hence the mixedEnergy BC through the e field).

To overcome this issue, the refValue can be set to an arbitrary value since the valueFraction=0 nullifies refValue's effect for these two modes.