add OpenVDB format

[orbeckst]

OpenVDB grids are used in Blender's volume grids. In order to make it easier to visualize biomolecular simulation density data in Blender, we may want to support export to OpenVDB.

A OpenVDB Python API exists.

There's a conda-forge package available:

• https://anaconda.org/conda-forge/openvdb
• license: Apache-2.0

[orbeckst]

See OpenVDB: Working with NumPy arrays how we can easily create the appropriate data structures and then write top a vdb file.