fixed a bug in morphological filters when voxel width≠1

This corrects the behavior of the "-dilation", "-erosion", "-opening", and "-closing" arguments to filter_mrc.

I forgot to take the voxel width into consideration when using morphological filters in filter_mrc. The thickness (width) parameter of the morphological filters was always being interpreted in units of voxels (instead of physical distance, as most other parameters are). Hopefully now this has been corrected. (You can still specify these thickness parameters in voxels, if you prefer that, by using the "-w 1" argument.)