pyopenvdb file streaming

### Is your feature request related to a problem? Please describe.
Currently `pyopenvdb` only supports writing to local disk. This prevents writing directly to a destination in some cases (e.g. to cloud storage).

### Describe the solution you'd like
The vdb read and write functions accept a file object (or pathlike object) instead of a `str`.

### Describe alternatives you've considered
Currently our workaround is to write to a temporary file on local disk and copy it's contents to the destination.

Start at the pyopenvdb vdb read and write entry points and trace how they currently require local paths. Determine how file objects and pathlike objects should be handled, then add coverage for both read and write cases, including a non-local destination; done means callers no longer need a temporary local file.

Written by the indexing model from the issue text.