Write fitted wavelength position and error to file

The existing 2D DRP code (IdentifyLinesTask) already estimates positions and errors of spectral lines, based on a reference line list. We need to write this information in the case of arc data, to a separate file.

More specifically, the fiberId, wavelength, line positions and error (y and delta-y) are written to an ASCII or FITS file, along with additional information identifying the arc exposure (pfsDesignId and pfsConfigId).

In addition, the CalibrateWavelengthsTask uses the information from IdentifyLinesTask to determine a wavelength solution for the detectorMap, flagging lines that are used as part of the fitting, or those that are reserved. That task also provides a revised estimate and error of the wavelength solution.

This additional information should also be written to the same file (there is some denormalization here but it makes more practical sense to carry both sets of information in the same file).

I suggest updating the method reduceArcTask.write() to do this, as it appears that most of the information is available there.