Facet-imaging updates

[jurjen93]

Update to the facet imaging workflow. The updates include:

• An option to provide extra averaging of the input MS (useful for lower resolution imaging). This needed a ValueFrom fix in the sort_mses step, because if the averaging is not used it returns an array with [null, null, ...], which first_non_null does not pick up as null.
• An option to provide the briggs weighting to the facet-imaging workflow
• An option to provide ncpu to the facet-imaging workflow for running WSClean imaging. If not given, the ncpu for WSClean is calculated based on the image size (scales between 8 and 64 cores in steps of 4).
• Fixes for label names
• Fix for warning when returning PSF images

[tikk3r]

I think this step is best combined with steps/dp3_prep_phasediff.cwl. That only does averaging as well.

[jurjen93]

Yes we can do this in a separate MR. Will add a ticket to the LOFAR-VLBI ticket board

[tikk3r]

I would carry finishing this PR over to the busy week then. It feels a bit moot to add a step and then immediately have another PR removing it already planned for the next weeks.