Automatic selection of delay calibrator candidate(s) based on phasediff

Draft because it strongly depends on the new split-directions workflow in !76 (merged), so getting that merged first would be best.

This MR is for https://github.com/LOFAR-VLBI/lofar-vlbi-pipeline/issues/12 of the GitHub project board. It relates to the plot at https://github.com/LOFAR-VLBI/lofar-vlbi-pipeline/wiki/2024-11-28 in the "LBCS & phasediff" section.

Selection of delay calibrators is still a challenging aspect. This MR adds functionality to the delay-calibration workflow to automatically select (the most) promising delay calibrator candidate(s), based on the phasediff score, which is also used to select the best direction-dependent calibrators. This MR adds a new option do_auto_delay_selection to the delay calibration workflow, which will run a new subworkflow find-best-delay-calibrator.cwl, which will:

1. split off all sources in the catalogue passed to the workflow
2. select the top N best candidates based on the phasediff score, set by select_best_n_delay_calibrators
3. run facetselfcal on those sources with models from skynet.py, or user-supplied text skymodels optionally passed to starting_skymodel

The original functionality has been retained, so if do_auto_delay_selection is false it runs phaseup-concat.cwl as usual. Skynet has also been updated to now be capable of generating a model for all sources in the list of delay calibrators instead of just the first.