CForest new version

Do not focus on the documentation, I am leaving it for the end since there are a lot of changes and probably more to come.
I would say this is not appropriate. Firstly, I am using back() and pop_back(). When RRTstar shares a new solution in a vector<states *>, this vector is organized from the goal to the start state. Actually statesToSample_ is a stack (but std::stack is based on vector or deque), so that when popping back we are doing it in the correct order (from start to goal). This is done in order to avoid reversing the path in RRTstar before sharing it.

However, the important point here is that the getNextSample() function is used in 3 different parts in this class. If we decide to change something, I would have to change it 3 times. I will do it a private method but, if you agree, I would keep it there.
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