Add option to use DG-type mesh geometry

[garth-wells]

This would:

• Test the flexibility/generality of the mesh geometry design
• Support contiguous memory access, at the expend of extra memory for storage
• Be a step towards supporting periodic meshes

[jorgensd]

Could you elaborate on the last point there, i.e. "Step towards supporting periodic meshes"? I don't quite see how that would be related.