DIC Physics: Differential Method

Implementation of the differential method from page 86 of
Thorsten's DIC book. Uses the image gradient to direct the search
for displacement. Includes shear, extension and bilinear interpolation.
Changes to get tests passing for new dic_method

Compute image mean and std_dev seem to be very sensitive to the
array limits when iterating pixels. i = 0 to image.width() is very
different than i=1 to image.width()-1. I'm not sure why this is,
but I have reverted the comptue_image_mean and std_dev back to the
old bounds for the time being.