AST-1168 Add XTensor view to DPBuffer
Use Google style for DPBuffer members.
Add DPBuffer::Resize* functions and adjust DPBuffer::MakeIndependent.
Replace code that modifies casacore fields directly by calls to DPBuffer, which updates the XTensor view, too.
This MR will merge into ast-1168-integration, which will contain follow-up changes, too.