Adds visualization on valid holes

It refers to valid holes from either RGB or Depth sources, or to the
merges between them.

	modified:   pandora_vision_hole_detector/cfg/hole_fusion_cfg.cfg
	modified:   pandora_vision_hole_detector/include/utils/parameters.h
	modified:   pandora_vision_hole_detector/src/hole_fusion_node/hole_fusion.cpp
	modified:   pandora_vision_hole_detector/src/utils/parameters.cpp