Add RMSynthesis workflow

[sposullivan]

Add the ability to process Stokes Q and U cubes with rmtools using rmsynth3d. This will generate the final data products for linear polarization. Make run_rmtools.cwl script in steps.

[sposullivan]

Added simple command to run rmsynth3d from RM-Tools for Q and U cubes requiring a input list of frequencies to scripts/run_rmtools.py
This script is linked to steps/run_rmtools.cwl and the workflows/polarization_imaging.cwl
This should be enough for testing once the Q and U cubes creation steps are present.

[sposullivan]

Basic implementation done and tested. Need to push back to github.