add zoom_2d: safe zoom without alias, and safe against inf/nan

add zoom_2d: safe zoom without alias, and safe against inf/nan

incorporate zoom_2d
add an optional parameter to homography_cropper indicating a subsampling
factor f to be applied to the output image. Thus the final homography will
be Z*H, where Z=diag(1/f,1/f,1)

added global_params still not zooming
add global parameter subsample_factor to rectify_pair:
if the factor z > 1 then the rectified images will be subsampled by a factor z
Merge branch 'master' of github.com:gfacciol/s2p

Conflicts:
	python/common.py