Flatten 3_D Jacobian into 2-D.

def jflatten(j):
    """
    Flatten 3_D Jacobian into 2-D.
    """
    nobs, nf, nargs = j.shape
    nrows, ncols = nf * nobs, nargs * nobs
    jflat = np.zeros((nrows, ncols))
    for n in xrange(nobs):
        r, c = n * nf, n * nargs
        jflat[r:(r + nf), c:(c + nargs)] = j[n]
    return jflat