Add basic example for Eigen

[MRAAGH]

The Ackley function example in the readme is very helpful for testing a fresh installation and getting started. Especially for people who have never used Armadillo before.

It would be nice to have something for Eigen as well. A simple example, without autodiff. Maybe even an equivalent Ackley function example but with Eigen?

[kthohr]

There are Eigen-based tests in the documentation files. See, e.g., here:

https://optimlib.readthedocs.io/en/latest/api/de.html#ackley-function