Update to openvdb 13.0.0

Highlights from this release:

• OpenVDB now includes in-memory support for Half Grids and the file format has been extended to add I/O serialization for half grids. More work is planned on adapting algorithms when using Half Grids for better performance and improved precision.
• NanoVDB is no longer limited to static applications, such as rendering, but can increasingly be applied to problems that involve dynamic topology, such as level set tracking, grid building, morphology operations and merging of grids. This is due to the fact that we keep adding more and more tools to NanoVDB that facilitates tree modification. However, unlike OpenVDB, these operations generate new NanoVDB grids (as oppose to in-palace modifications in OpenVDB), yet are typically much faster since they are performed on the GPU. Many of these exciting new features are the foundation of fVDB.

See the release notes for more details.