Include condensation flux consistently into the energy balance

[nichollsh]

Condensation is important as it prevents the temperature dropping below the dew point temperature. Including this in the time-stepping solver is easy, but including it into the nonlinear solver is more tricky.

Maybe one way to do this is to set the power dissipation (i.e. F_bot-F_top across a cell) to be strictly positive when T<=T_dew.

[nichollsh]

This has been partially implemented in the condensation branch. It uses a method for 'relaxing' the temperature profile onto the saturation curve, but introducing a signed residual when T<T_dew which forces T=T_dew to be a valid solution to the system of equations.

[nichollsh]

Closed by #29.