hdf5_msb

I've changed `test/dsets` so that every time it needs to visit all
cells in a matrix in an arbitrary order, first it chooses a random
starting `offset` in [0, rows * columns - 1].  Then it chooses a
random `increment` that's relatively prime to `rows * columns`.
Then it visits every cell in `rows * columns` steps:

        for (i = 0; i < rows * columns; i++) {
                visit(cell[offset / columns][offset % columns]);
                offset = (increment + offset) % (rows * columns);
        }

By moving the HDrandom() calls outside of the main loop and visiting
each cell only once, this probably speeds things up quite a bit.  It's
also more resilient to a crummy random sequence.  The new code visits
cells in an order that's probably arbitrary enough for testing purposes.