Basic helper script to convert a set of (x, y, z) coordinates to (i,
j, k) indices for a dset.  Essentially stealing sage advice written by
DR Glen in a helpful Message Board post.

Run this program by entering exactly 4, space-separated arguments:
    the name of a file, and then 3 coordinates (x, y, z).

Program returns 3 indices:
    i j k
(which can be redirected into a variable or file, for example).

If any of 'i j k' are outside the dset's matrix, return an error.