Feature proposal applySubtreeVisitor()

[galou]

In order to allow the combination of C++ (action servers) and BehaviorTree, we preferred to rely on copying the required C++ variables to each subtree's blackboard rather than using _autoremap. My solution was to implement applySubtreeVisitor, cf. 0843158. This cannot be merged as-is because I remove the applyVisitor function rather than deprecating it but this is easy to change.

@facontidavide what do you think about this?

[facontidavide]

not sure about this design pattern, in general

[facontidavide]

not doing this, for the time being