Move the polar decomposition to a separate file and add the algorithm by...

Move the polar decomposition to a separate file and add the algorithm by Higham and Noferini (from Robin Fraenzel)

In the test for the Higham and Noferini algorithm, we see:
Unfortunately, for matrices that are close to an orthogonal matrix, this algorithm is
about 2x slower. One can benefit from the Higham and Noferini algorithm only if the
matrix is quite far away from an orthogonal one.