wsclean

Created by: AlecThomson

New feature - adding wsclean as an imager!

I've added a wrapper function which exposes all the options of wsclean. I've added some options to the default config, but not all are really necessary. Where appropriate, the common params between tclean and wsclean are used. Its important to note, though, that useful defaults for tclean may not be appropriate for wsclean and vice-versa.

For this to work, it requires a separate installation of wsclean and its libraries. Also, to be compatible with IDIA-pipeline MSs v3.0 of wsclean is needed.

wsclean does it's own parallelisation using wsclean-mp and some internal threading. This means that when using wsclean, the split stage is not needed. The advantage of wsclean is that it can do joint-convolution, using the full bandwidth when deconvolving.

This PR builds on the updates of #4 - so we may want to clear those changes first before merging this one.