Adding parameterizations of Hurwitz matrices

[Alex-Hache]

I added a parameterizations of Hurwitz matrices, i.e. the one that have eigenvalues with negative real parts they represent stable linear dynamical systems. Optionally one can enforce a specific bound on the matrix decay rate named alpha, it is related to the concept of alpha-stability in Lyapunov linear theory.

In order to build this parameterization, I changed the constructor of the Skew class that now takes a size parameter. Now the parameterization is instantiated with _register_manifold instead of register_parameterization. The test_skey.py file has been modified accordingly.

(This is my first PR ever do not hesitate to come back to me if something does not meet the requirements)