Trace shifts in pipeline

[julienguy]

Include the fit of trace shifts to the pipeline (issue #487)

• needed because of spectrograph trace shifts with temperature
• the fit depends on the "flavor" of the input preprocessed image.
  • for continuum lamp exposures: only along x
  • for science exposures, along x, internal (fiber to fiber) wavelength recalibration ; then global wavelength recalibration on reference sky spectrum.
• inserted between psfnight and extraction in the task chain.
• tested at scale at NERSC