Topic: Exporting primary fields should use.. PrimaryField.

The nodal averaging thing we use in VTKXML must go. It's really meant to be used with internal state types, but here it's reused for the primary field as well.

It has problems. As far as I can tell, VTKXML doesn't export displacements in global coordinate system. We need to use a PrimaryField here instead.

We need to ensure that the primaryfield can export values in the situations like:
1. When the nodal values exist, use them *in global c.s.*
2. When nodal values don't exist, use element interpolation to determine value (typical for mixed interpolations).
otherwise, export zeros (since we must always export something in VTU.

It shouldn't be achieve this.