ArcDark identification throughout pipeline

[Nat1405]

Noting that the isArcDark check in nifsSort.rewriteCalibrationLists() doesn't use exptimes brought this up. This is a problem because arc darks are currently identified by checking that their exposure time is the same as a known arc. It would be good to go through the pipeline and make sure that arc darks of the proper exposure time are being used (it might be fine though, since we're only using the arcs for a wavelength calibration).