fixed bugs in "combine_mrc" and the "-normals-file" argument, and added the "-undefined-out" argument

Several changes that probably won't have a big impact on most users.

• Fixed a bug in "combine_mrc" causing thresholding to fail (which was breaking the NAND and NOR examples).
• Fixed a bug so that the "-normals-file" argument (in "filter_mrc") now works better with masks.
• Simplified and generalized the API for "Watershed()" and "ClusterConnected()" (in "segmentation.hpp" and "clustering.hpp").
• Added an "aaaiMarkers" argument to Watershed() (so that it now matches all the features in the scikit-image version). (This feature is not yet available in the "filter_mrc" program.)
• Added the "-undefined-out" argument to "filter_mrc".