Adds BlobDetection::brushfireKeypoints test

Also, the BlobDetection::brushfireKeypoints method now searches in a
cross-expanding way for pixels of zero value.

	modified:   pandora_vision_hole_detector/include/utils/blob_detection.h
	modified:   pandora_vision_hole_detector/src/utils/blob_detection.cpp
	modified:   pandora_vision_hole_detector/src/utils/hole_filters.cpp
	modified:   pandora_vision_hole_detector/test/unit/utils/blob_detection_test.cpp