Improve Thigmotropism Performance/Quality

[B00merang]

Issue: Thigmotropism runs at below interactive rates, even for simple plants.

Cause: The Volume initialization step is heavy

Solutions:

1. Don't create a new volume every step, update instead
2. Don't used a single fixed resolution regardless of size
3. investigate ways to speed up the vdb operations
4. Ditch VDB operations entirely and use a point field with xyzdist/nearpoint operations
5. customisable growth frequency