can now search for non-spherical blobs (aligned with the XYZ axes)

I added the "-blob-aspect-ratio" argument to filter_mrc. This new argument allows users to search for blobs that are elongated in the X,Y,Z directions. This also required changing the API for VISFD. The "BlobDog()" function (in the "feature.hpp" file from the VISFD library) now accepts a new argument ("aspect_ratio").

This is not a true anisotropic blob-detector, because the blobs cannot be arbitrarily oriented. It is assumed they will be elongated in the X, Y, or Z directions .