this is required because temperature needs to be always specified. in
the case of isothermal simulations, the temperature is assumed to be
the initial one, i.e., freeing up the initial fluid states also makes
the temperature undefined.
I suspect that the reason why this did not lead to crashes is that for
isothermal `BlackOilFluidState` objects, the temperature is stored in
a static member variable.
thanks to at [at]bska for catching this issue.