Model::computeFullTreeTraversal crashes on iDynTree::Model with loop

[traversaro]

Even if it is not supported in most classes for kinematics or dynamics computations, iDynTree::Model supports representing graphs with loops. However, if a iDynTree::Model with graph with a loop is passed to Model::computeFullTreeTraversal , the method crash. Instead, the method should either print an error or compute the traversal corresponding to a spanning tree.