More robust join bundle node

Edit: to say that this is actually a much bigger deal than it appears. It removes the ability to join dynamically named bundles in a lot of instances, basically rendering the join bundle node almost useless.

Currently, the join bundle node cannot join nested bundles if they have the same elements in them.

For example: If you have the following structure in a bundle:

• root/leaf1/element1
• root/leaf1/element2

and you have a second bundle with this structure:

• root/leaf2/element1
• root/leaf2/element2

and you try to join them, Blender will complain about duplicate keys and give you the following result:

• root/leaf1/element1
• root/leaf1/element2

Essentially, only the first bundle is kept because there is a collision of keys at the very top ("root").

But in practice, there is no collision because those are directories and not actual data elements. It should be trivial to merge these two. If a directory already exists, do not create a new one. Use the existing one to populate with elements. If a directory does not already exist. Create it.

Only actual data elements should trigger a duplicate keys warning, and in those cases the current behavior (keeping only the first occurrence) is warranted.