and since it was my first try of the patch queue feature I did not
managed to apply it with a good commit message, so here you go:
* Add a ComplexSchur decomposition class built on top of HessenbergDecomposition
* Add a ComplexEigenSolver built on top of ComplexSchur
There are still a couple of FIXME but at least they work for any reasonable matrices,
still have to extend the unit tests to stress them with nasty matrices...