Allow adding facets with masks

This MR adds functionality to add a facet image to the larger image with specific weights, thereby allowing feathering.

To feather, WSClean would first call MakeMask() to get a non-feathered mask; run this through a tophat convolution (this MR in WSClean) and then use AddWithMask() to add the facet data to the main image while using the feathered mask.