pc_align: Some speedup using multithreading
build_vw_asp: script to build ASP/VW
build_vw_asp: default source dir change
sparse_disp: minor changes + better handling of failed matches

Gaps where the epipolar test failed are now marked in D_sub_spread with zeros.
Other improvements:
-- only 25% of the search window must be marked valid for a search to occur (had been 50%)
-- the valid mask is diated before writing or before being used to mask D_sub_spread
-- initial search pts now can be as little as coarse_skip/4 from the image edges
-- A larger percentile fraction of disparites are used in calculating the epipolar vector
sparse_disp: fix minor bugs

small typos fixed
sparse_disp: check and clamp point indices
build_vw_asp: new dependencies for pc_align
build_vw_asp: need eigen include flag
sparse_disp: revert clamp fix
sparse_disp: fix gdal import line
sparse_disp: clamp point x and y indices
sparse_disp: no exception for std_t = 0 cases
sparse_disp: auto coarse_skip determination
sparse_disp: retab
sparse_disp: add print for initial search
sparse_disp: default output_scale 8
sparse_disp: clean up im_subset class, add new methods