Enable auto-mask mask compression

Using this in WSClean and running on a typical field but very small image, resulted in the last auto-masking iteration in:

Compressing scale dependent masks...
Compression rate of scale mask 0: 436.9x
Compression rate of scale mask 1: 3072x
Compression rate of scale mask 2: 2137x
Compression rate of scale mask 3: 2048x
Compression rate of scale mask 4: 391.1x

In earlier iterations it's even higher, and I expect larger images will also yield higher compression rates.

Be aware that this MR only makes sure that the auto-masks are compressed (not the regular masks), and also only when using parallel deconvolution. These other use-cases could be dealt with in later MRs, but that is for later. This MR implements the biggest savings in the way we run this in Rapthor, because there we use auto-masking and parallel deconvolution.

When building WSClean against this MR, the following is an example of a run that would make use of this compression (and produced the output above):

wsclean -v -size 768 768 -scale 45asec -niter 1000000 -mgain 0.8 -multiscale \
  -auto-mask 2.8 -auto-threshold 1 -parallel-deconvolution 512 -use-wgridder ../1052736496-averaged.ms/