In preprocessing, compute trace shift prior to estimate background between fiber bundles

[julienguy]

• When running with option --bkgsub-for-science, the preprocessing code uses the CCD pixels between fiber bundles to fit a residual bias correction. This is in place in the pipeline for the blue cameras. This procedure fails if the fiber trace coordinates in the default PSF are off.
• We should modify the preprocessing code to include a fit of the shift of traces for this background estimation.
• Note that traces shifts are fitted when preprocessing is run with option --model-variance, so we should make sure to run this fit only once when both options are set (which is the case in the pipeline for the blue cameras).