Python VDS creation doesn't work

There are two problems:

• The VolumeDataAxisDescriptors created by Python end up pointing their names and units to temporary copies of strings that are then destructed. This leads to invalid strings for names and units for the created VDS.
• The GetBuffer()/GetWritableBuffer() calls on VolumeDataPage don't create buffer views that actually work with Python and you just get a raw pointer that you can't use for anything.