Clean up aitchison a bit.
Make simplex_grid more flexible.

Details:
    `depth` and `base` are removed, in favor of `subdivisions`.
    Old behavior:
        simplex_grid(3, 4, base=2)
    New behavior:
        simplex_grid(3, 2**4)

    Also, now you can pass `using=np.array` to receive arrays back.