Add methods to switch vtkCellArray default storage at runtime.

Add a static bool and method `SetDefaultStorageIs64Bit(bool)` to
change the default storage used by vtkCellArray to 32 or 64 bit.
This supports a use-case where most models are small enough to use
32bit internal storage, saving memory, but if a large model is
detected on load, the default can be switched to 64bit going forward.