Spy plot with partitioning.
SPYPART(S,rp,cp) plots the sparsity pattern of a matrix S,
with lines marking a block partition described by
rp (rows) and cp (columns).
If S is square, cp may be omitted and defaults to rp.
Partitions are specified as in the output of DMPERM :
There are length(rp)-1 row blocks, of sizes diff(rp), with rp(1)=1.