Fix output of VFS on interval
Fix output of HDiv/HCurl fields

The test for if they were CG or not was wrong.
Fix DG coordinate field for periodic interval

Ugly hack until we get fed it from DMPlex.  Note that this is probably
wrong when symbolic geometry stuff lands.
Add some simple div-free tests for extruded intervals
Merge pull request #211 from firedrakeproject/fix/periodic

- Fix the coordinate field for periodic interval
- Fix some corner cases in VTU output