Changes NanoVDB to be a standalone dependency that is independent of the OpenVDB one.
It works by downloading the "feature/nanovdb" branch of OpenVDB, but using the NanoVDB
CMake in the "nanovdb" subdirectory. Since it is header-only, only the install target is used.
Reviewed By: brecht
Differential Revision: https://developer.blender.org/D9383
