Improved treatment of ILWR
- Force incoming LW from the flat field for the virtual slopes.
- If LW_NET is provided, and in addition also a TSS, calculate incoming LW.
A final check is necessary if this merge request isn't making the key MEAS_INCOMING_LONGWAVE ineffective. This is fine for the cases where we have a reasonably good estimate of ILWR on the flat field (measured, modelled, paramaterized), in which case MEAS_INCOMING_LONGWAVE is recommended to use the same ILWR for the virtual slopes. But I think we need to cover cases where TSS is used to force the flat-field simulations, there is no reliable ILWR, and we want to have the virtual slopes get some information from the flat-field. In that case, MEAS_INCOMING_LONGWAVE should be set to false, and the LW_IN / EA from the energy balance closure on the flat field that is informed by the measured TSS should be used for the virtual slopes. The latter has to be tested if it still works with this merge request.