CAS-14719: avoid small ragged trailing block in cube minor cycle partitioning
numblockchans derived blksize from nproc and then took nblk=nchan/blksize,
truncating the division; the remainder was re-appended as an extra, much
smaller trailing block. nblk was routinely nproc+1 and the orphan block
could hold only a few channels. The deconvolver derives its mask-smoothing
beam from the block's own beam subset, where getCommonBeam returns
trivially for a single-beam set but enters ill-conditioned arithmetic for
a few very similar channels. Decide the block count first, rounding up so
no block exceeds optchan, then split the channels evenly.
