nan-padded voxels have g/n=0

[lkeegan]

Single photon debug runs with a nan-padded layer at z=0:

With b=0 the photon exits voxel [93 12 0]:

b0.log

With b=1 the photon is instead reflected, with the message ref total ref=-inf:

b1.log

With the change from #224 the b=1 case above becomes the same as b=0. However a full simulation gives many errors involving photons at this z=0 layer:

ERROR: should never happen! mediaid=0 idx1d=88F isreflect=1 gcfg->doreflect=1 n1=1.299805 n2=1.299805 isdet=0 flipdir[3]=2 p=(31.562723 18.700874 1.000000)[31 18 0]
ERROR: should never happen! mediaid=0 idx1d=2A6 isreflect=1 gcfg->doreflect=1 n1=1.299805 n2=1.299805 isdet=0 flipdir[3]=2 p=(78.180321 5.373124 1.000000)[78 5 0]
ERROR: should never happen! mediaid=0 idx1d=11E isreflect=1 gcfg->doreflect=1 n1=1.299805 n2=1.299805 isdet=0 flipdir[3]=2 p=(46.498226 2.525769 1.000000)[46 2 0]
ERROR: should never happen! mediaid=0 idx1d=B1 isreflect=1 gcfg->doreflect=1 n1=1.299805 n2=1.299805 isdet=0 flipdir[3]=2 p=(57.182274 1.923315 1.000000)[57 1 0]
etc