OpenVDB Install Changes #53

I ran through the install instructions under the README, but had some issues specifically with the OpenVDB setup/install. Easily worked around, but figured I'd drop what I did under an issue for future reference.

The issues/workarounds:
1 - The command

cp openvdb/openvdb/python/pyopenvdb.* `python -c 'import site; print(site.getsitepackages()[0])'`

in the README fails because there ends up being no objects matching the naming scheme pyopenvdb.*. At least for me, the .so file I assume you're looking for is named openvdb...so instead. Not sure if that's a change from OpenVDB or what but figured I'd mention it if you want to update the docs.

2 - Because of that, I had to change the import statement here to import openvdb as vdb instead of import pyopenvdb

3 - If for some reason you're using a Python version built for x86 on Apple Silicon, which I was doing in the venv I happened to be using at first, then the OpenVDB bindings won't work when built with the current instructions. Maybe there's a way to change the build behavior to make it work, but I just made a new venv with an arm Python version for myself. This is only a problem if you're using the mesh module of the library AFAICT.