This adds API to create memoryview objects for the VolumeDataPages so it is possible to write data to a VDS using Python.
- Add API to get the VolumeDataPageAccessor from a VolumeDataPage.
- Make sure we include our own pybind11 version before anything installed on the system.
- Avoid releasing the GIL when using py::cast.
- Create Python memoryview objects for VolumeDataPage GetBuffer/GetWritableBuffer methods.
- Add 'unitless' to the KnownUnitNames.
- Add more code to the Python createtest so it writes to the VDS and checks if the result is correct.
This resolves #73 (closed) "Python VDS creation doesn't work".