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.