Reconsider the scaling of fluxes in pfsMerged, and the use of Jacobians in interpolation

arnaud.lefur pointed out a possible bug in the conversion of pfsArm to pfsMerged fluxes (where we convert from electrons to electrons/nm, and resample to a common grid). Here I will reconsider and/or attempt to justify the current scaling and resampling.

It's worth pointing out that none of this affects the final flux calibration of pfsSingle and pfsObject (because those have applied a separate scaling using standard stars), only the scaling of pfsMerged.