Compute correct ISes for parallel mixed vecscatter

A MixedDat of k Dats on P processes is laid out on process i as:

    (x_0_i, x_1_i, ..., x_k_i)

The correct ordering for multiplication by a MixedMat is:

    (x_0_0, x_1_0, ..., x_k_i, ..., x_0_P, ..., x_k_P)

Previously we produced

    (x_0_0, x_0_1, ..., x_0_P, ..., x_k_0, ..., x_k_P)

instead, which led to incorrect results when running in parallel.