Tangent update is symmetrical.
When adding tangent 2 handle, use its own value description, regardless the splits flags
Typo
Deprecate set_split_flag. Use the other flags.
When set a flag only merge tangenets if the flag has changed from true to false.
When modify one tangent update the other. Not working well yet.
Duckmatic: do not update the tangent that is being changed. Clean up debug code
ValueDescSet: When modify one tangent value node, update the other but not recursively.