sparse_disp: add two-element input arguments to trim method for im_subset
sparse_disp : change output correlation value for failed correlation

useful in debugging
sparse_disp: added single-thread option

useful for debugging: specify a negative number of processes to get a single-thread run
sparse_disp: fix sparse matrix formats

need to convert matrices before editing to avoid a warning
sparse_disp: write output tifs in blocks

output files are now written in blocks of ~8k pixels.  Avoids some memory problems for some very large input images