added many small features to filter_mrc

added the "-blur-expand", "-blur-contract", "-mask-rect", "-mask-rect-subtract", "-mask-sphere", "-mask-sphere-subtract, and "-fill" arguments to the "filter_mrc" program. They have been debugged and documented.

I also fixed a bug in the "-image-size" argument, as well as a bug in the underlying MrcSimple class library (in the "lib/mrc_header.cpp" file). (There may be bugs in the way the "origin" attributes from the MRC header are read and stored. I currently ignore these "origin" numbers and set them to 0,0,0. But we may have to worry about them one day. Until then, ignorance is bliss.)