Properly fix moments for OpenCV single-pixel / single-line segments

[Tobias-Fischer]

At the moment, there is an issue where self.data and contours do not have the same number of list elements, leading to a crash:

machinevision-toolbox-python/machinevisiontoolbox/ImageBlobs.py

Lines 256 to 260 in dd408f2

	# TODO: this should recurse all the way down
	M = blob.moments
	for child in blob.children:
	# subtract moments of the child
	M = {key: M[key] - self.data[child].moments[key] for key in M}

We previously tried to address this in #5, but this was an incomplete fix. Here, we go through the TODO and properly set all the moments where OpenCV returns nonsense.