Add .in_volume() method to neuron classes

[schlegelp]

This requires not only a thin wrapper for morpho.in_volume but also removal of connectors and more importantly closing of branches that don't have a parent anymore. There may be quite a few functions that act up when there is more than one root node.

[schlegelp]

Along the same lines: .add_annotation() .remove_annotation() .clean_up() --> this should require additional confirmation by user!