Update sandwich hand detector so it can also check for the black outline

[jw098]

Updated the detector to reduce false positives on lettuce. It checks that the matched waterfill object contains the black outline.

• for waterfill objects that matched the hand template, apply a mask filter to retain all black components
• if this is a true positive, waterfill_sub_binary_image should be a black outline of the hand.
• if false positive, it will be a pure white image
• then apply the waterfill+template match algorithm on the resulting waterfill_sub_binary_image
• this helps to remove false positives (e.g. lettuce)

SandwichHandLocator::detect will still use the old method of detection, for performance reasons (I didn't test it, but I imagine that adding an extra round of waterfill matching will worsen the lag in an already laggy program). So, I think that detecting the black outline should only be done for recovery routines.

I recommend merging #428 first.

[jw098]

Closing for now, since this doesn't seem to be an issue with the new sandwich hand template.