Add polarization workflow

[vijaymahatma]

Make a CWL workflow that runs QU imaging, concatenates the images, and makes linear polarization images using rmtools.

[vijaymahatma]

add_qu_imaging branch created for all polarization work.

New CWL workflow added that

1. Runs WSClean to make polarization images (full Stokes or QU only)
2. Concats the channel images into cubes for each Stokes parameter
3. Runs RMSynthesis with rmtools3d to produce polarization imaging products.

Verification and testing of associated subworkflows required before testing this workflow. https://github.com/LOFAR-VLBI/pilot/blob/add_qu_imaging/workflows/polarization_imaging.cwl (validated with cwltool))

[sposullivan]

I got a version to run end-to-end so we just need to figure out how best to push this back to the github for others to test and improve