Add support for pip install

[matthewdcong]

This PR adds preliminary support for pip install. Assuming that the dependencies required for building OpenVDB and NanoVDB are installed, then pip install <openvdb_source_root> will

1. Build OpenVDB, NanoVDB, and their corresponding Python bindings.
2. Install these Python bindings and dependent libraries in the site-packages directory
3. Enable the user to import openvdb and import nanovdb when launching Python.

This is a first-step towards making installation of OpenVDB easier for new users, and will help users dive into the library via the Python interface.