Include feather boundary in facet size

This merge requests adds a -feather-size option to WSClean and propagates the value to the facet calculations, such that the bounding boxes are increased by the required size. If the option is not specified, a (first) default is selected of 1% of sqrt(width * height), e.g. for a 10K x 10K image, this results in 100 pixels of overlap -- which to me seems reasonable, but this will require more experimentation to see if this is overall a good setting. After this MR, the facet boundaries are increased. Everything keeps working, but it does of course slow down imaging -- so far with no benefit, because the facet data is directly copied to the full image, without feathering. In a next MR, the feathering will be added.

Requires https://git.astron.nl/RD/schaapcommon/-/merge_requests/115 to be reviewed and merged first.