PfsArm needs additional information to recover raw counts

The datamodel.txt information for the pfsArm object mentions that 'the pfsFiberTrace file contains enough information to recover the raw counts and the widths of the fiber traces'. However, this is not currently possible as the pfsArm object only contains a FLUX HDU (in units of nJy), but no additional calibration vector or equivalent to recover the raw counts.

It may not even be necessary to store the flux in nJy at this stage, as flux calibration is performed later in the pipeline.

Please add an additional HDU to this object such that raw counts can be derived. In addition consider whether the FLUX HDU really needs to carry absolute fluxes, or store 'flux' measurements relative to the illuminating quartz lamps.