Fixes for contact-vi-foo demos.

Demos that rely on a particular mesh structure should provide a mesh from file. They should not rely on in-built mesh generation.
Modify periodic demo to test Issue #71.

I can't reproduce Issue #71. Maybe problem has been fixed in Swig?
Use FFC '-f split' option for convergence benchmark.
Merge remote-tracking branch 'origin/garth/split-convergence-forms'
Merge remote-tracking branch 'origin/garth/fix-issue-71'
Merge remote-tracking branch 'origin/garth/fixes-snes-vi'