Implementing CtcCartProd the cartesian product of two contractors

[Teusner]

Implementing the cartesian product of two contractors.

TODO:

• [ ] Extend the mechanism using variadic templates to cartesian product of n contractors
• [ ] Add an example in the documentation
• [ ] Python binding : How to manage template or variadic template with pybind11

[SimonRohou]

Thanks! Commits have been merged in dev with Python binding + Python tests.

Tests are running well on my computer, but not here: https://github.com/codac-team/codac/runs/6388570720?check_suite_focus=true

For the documentation, explanations about the involved symbols would be nice, with a small figure :)