Convert pfsMerged fluxes to electrons/nm

pfsMerged fluxes are currently in electrons per wavelength sample (since the wavelength sampling is similar to the original sampling, the fluxes are roughly in units of electrons per pixel like in the pfsArm files). We agreed in the Princeton meeting today that it would be better if the pfsMerged were in units of electrons/nm.

• Change the units of the pfsMerged fluxes.
• Check that this doesn't break the calibrated fluxes in the pfsObject files.
• Document the flux units of pfsArm and pfsMerged in datamodel.txt.