NanoVDB file version discrepancy

This time, converting an OpenVDB file to NanoVDB format and trying to read it with the same revision (1c306bcaf7e9f7d52525bab716202bd4f7fff5d1) outputs an error:

```
Incompatible file format: The file is written in an older version of NanoVDB: 27.0.X!
Recommendation: Re-generate this NanoVDB file with the never version 28.X of NanoVDB
```

Reproduce the OpenVDB-to-NanoVDB conversion and read attempt using revision 1c306bcaf7e9f7d52525bab716202bd4f7fff5d1. Compare the file's reported NanoVDB version with the reader's expected version; done means a file generated and read at the same revision is accepted without the incompatibility error.

Written by the indexing model from the issue text.