Improve facet region construction

This MR improves the way the boundaries of the facet regions are constructed. The new way ensures that the regions always encompass the full image, avoiding problems in WSClean that occur when the regions do not fully cover the image. It also ensures that all calibrator patches are included in the facet regions, as WSCLean requires this even if a patch lies entirely outside the imaged area.

A small tweak was also made to the filter_skymodel.py script that runs after WSClean to shorten processing times somewhat.