prune_and_map

Usage:
    <prune_and_map a b f
finds a minimal presentation b for coker a and a map
    f: target b --> target a
inducing the identity on the cokernel.
Parameters:
            a = matrix
Output values:
            b = minimal matrix with the same cokernel
            f = matrix so that fb = ag for some g and f induces 1 on
                            1: coker b --> coker a.