Improve installation layout on Windows for openvdb_houdini and python

In writing the *Install* steps in my [comment](https://github.com/AcademySoftwareFoundation/openvdb/issues/603#issuecomment-580097614) for #603, I needed to rejig the installation layout and would need to do less with some improvements:

- openvdb_houdini needs to install into a bin directory, similar to how openvdb is done because the DLL library search order is based on the PATH on Windows (as opposed to LD_LIBRARY_PATH on Linux).
- On Windows, python extension modules need to be renamed to a `.pyd` suffix instead of `.dll`. There might be an easy CMake way to directly build it with such a filename extension.
- For Houdini, the python extension modules need to be installed into `$HOUDINI_USER_PREF_DIR/python2.7libs` which corresponds to `OPENVDB_HOUDINI_PYTHONRC_DIR` in `openvdb_houdini/CMakeLists.txt`.