Write pfsArm in electrons

It would be useful for engineering and debugging pipelines if the extracted spectra in the pfsArm files were in units of electrons; currently they are relative to the fiberProfile normalisation, which comes from extracting a quartz). Similarly, it would similarly be useful if the spectra in the pfsMerged files were close to electrons (or, at least, normalised like they are close to electrons).

This will require small documentation changes in the datamodel, and some modest changes to the pipeline to support.