Not usually used on its own, called from within dsm.var.movblk.
generate.mb.sample(num.blocks.required, block.size, which.blocks, dsm.data,
unit.info, n.units)number of blocks that we need.
number of segments per block.
which blocks should be sampled.
the $data element of the result of a call to dsm.
result of calling block.info.per.su.
number of sampling units.
vector of log-residuals