Robust remove() API

I experienced a strange behavior that took me a lot to understand due to a not robust API for removing wolf nodes.

There is an automatic removal of the parent node if it ran out of children. For instance, when the last feature of a capture is removed, the Capture::remove() calls Frame::remove(). This is quite invisible by the user that just typed capture->remove(). And it ends up in this very strange behavior in some specific cases of the ProcessorTrackerFeature workflow.

Protections

1. For now, I added an assertion in all wolf nodes link() function checking that is_removing_ == false: you can't link a node that has been removed (even if you have its pointer so then it is alive..).
2. The functions getChildrenList() are returning via non-const reference. Then, the list of children of the nodes can be modified from the outside. I switched them to const reference return.
3. We have to control this automatic empty parent removal. One solution could be adding a bool parameter in the function: remove(bool viral_remove_empty_parent=false) that performs this automatic removal of parents (viraly) only when it is true.