Drift correction robustness, docs update,

[vreuter]

• Close #173
• Update pipeline execution control doc after discussion w/ @TLSteinacker
• Plotting spot counts for regional and for locus-specific FISH probes
• Update "bead_points" / num_bead_points to num_bead_rois_for_drift_correction (now both config key and attribute on ImageHandler)
• Label per-probe plots as "probe" rather than "timepoint" -- close #202
• Various helpers and combinators for working with Sets (collections.scala) and with JSON data (UJsonHelpers.scala)
• Adding failCode to detected ROI type (DetectedRoi), preserving that value from the bead ROI generation phase