Treat dual-band BSDF as single band for single layer systems

[StephenCzarnecki]

For systems that only have a single layer consisting of a dual band BSDF when the optical method is SOLAR or PHOTOPIC create a single band material with the appropriate band values. In all other cases create a dual band material as before. This is so that solar and visible results for a single layer dual band BSDF give results that match the band values specified in the input file. For more background consult @jcjonsson

[StephenCzarnecki]

This should be fixed as a result of b4b2da1 and b9ecfad. Expected results are updated here 9f6516d