sparse_disp: remove testing and SSD code for the norm_xcorr code
sparse_disp: change epipolar test

epipolar test now compares ep_dist to the maximum of ep_tol and 0.02*abs(disp-median_disp).  This lets us use a smaller default ep_tol and seems to give cleaner results
sparse_disp: allow a low-res mask file to prevent matching for some pixels

Can now specify a lowres mask file .  Nonzero entries mean that sparse_disp will try to mask a given pixel, zero means skip.  Geotransforms are handled so that the mask can be of arbitrary resolution
sparse_disp: move gridding to its own routine, allow an optional argument to specify the dilation distance for the valid mask

Gridding the output disparities was pretty ad hoc.  Now there's a formal process to ensure that disparities and spreads don't get interpolated too far, and to check the distance between the grid points and the data points.  An optional argument can specify the dilation of the mask around the valid data points.