Generalizes `numpy.vstack` to :class:`Matrix` objects.

def vstackm(matrices):
    """Generalizes `numpy.vstack` to :class:`Matrix` objects."""
    arr = np_vstack(tuple(m.matrix for m in matrices))
    #    print(tuple(m.matrix.dtype for m in matrices))
    #    print(arr.dtype)
    return Matrix(arr)