Extract incremental logic from TD3

[sim642]

To be done after #391 is merged.

• Unmarshaling etc should be done outside (by Control?)
• Finding nodes to destabilize due to change should be done outside (no node type hacks in TD3) Collect sets of constraint unknowns corresponding to program variables, functions and nodes #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?)