- Move some security checks outside of interp callbacks. Namely, that we do get interpolation weights, and have something to interpolate. Some callbacks where not checking on those anyway, safer to move that up into calling code.
- Cleanup usage of sub-weights, lots of interpolation callbacks wher actually using those completely wrong.
- Change default behavior when no weights are given to higher-level API functions: prevriously, each callback was responsible to handle that case (and one did not even do it!), they were switching to purely additive behavior then. Instead, we now default to expected simple average of source values.
Note that the only real important change here is defaulting to actual
average of source value when no inertpolation weights are given (afaik,
this only happens in Weld modifier code).