Minor updates and bug fixes for sparse_disp

These changes help fill some of the gaps that sparse_disp had been leaving, and also prevents a few bad zero-dispairty matches from leaking through.  It also adds a debugging option that writes out csv files that are handy for figuring out where holes in the D_sub come from.

Updated 11/14-- the newest commits in this branch reduce the really huge memory use that sparse_disp was building up when dealing with long, full-res WV strips.