Normalisation of Parareal update matrix for non-scalar problems

[danielru]

For problems with more than one component, e.g. shallow water equations, the update matrix is an actual matrix and not just a scalar. The normalisation procedure in this case needs to compute the n-th root of this matrix. So far this function is only implemented for scalars.

[danielru]

Write script to test normalisation for a 2 component acoustic-advection system.