Termination constraints to IK

We need a generic interface to IK constraint (termination ok or not) in timor. I came across this challenge when implementing follow goals with constraints. I think it makes sense to split them up into constraints that are checked during numeric iteration (if applicable) [like the one I added in this MR] and constraints checked after convergence [like the current implementation of collision checks].

Do you have any thoughs on the structuring of such an interface @MatthiasMayer ? I will continue working on this as soon as I have a clear understanding about the requirements.