Adding Constraints

[SiruiXie]

Great work @Levi-Armstrong ! Really helped me a lot.
I was trying to add some constraint to the optimization process, and by default, not using ifopt, but I find it hard to understand where should be the interface to add that constraint, take basic_cartesian_example for instance. Also, I was wondering if there exists an interface to add any constrains between different frames, or time_steps, in a trajectory.
It would be appreciated if I could get any reply on this issue. Thanks a lot!