Extract incremental logic from TD3

[sim642]

To be done after #391 is merged.

• [x] Unmarshaling etc should be done outside (by Control?)
• [x] Finding nodes to destabilize due to change should be done outside (no node type hacks in TD3) #544
• [ ] TD3 should be non-incremental, TD3-incr should wrap it with pre- and post-processing.
• [ ] Experimental TD3 improvements should go to a new copy (TD-inter?)