fixed two bugs in the "crop_mrc" program

This small update fixes two bugs:
-fixed a crash which occurs if the xmax, ymax, or zmax arguments are set equal to the size of the image (their maximum allowed values is the size in that direction -1). Now the program handles this user error gracefully and prints a warning message.
-fixed (hopefully) a bug in the way the "origin" was being modified during the cropping process. (The "origin" is one of the fields in the header portion of the MRC file. It will be modified whenever the xmin, ymin, or zmin arguments are non-zero.)