added the "-markers" command line argument to "filter_mrc"

This allows "filter_mrc" users to specify their own custom markers for use with watershed segmentation. This was an attempt to replicate the "markers" argument of the skimage.morphology.watershed() function. (Although I have not actually verified that the two codes behave the same way.) I also fixed a bug in the "-find-minima" and "-find-maxima" arguments. I also continued cleaning up the messy VISFD API.