Periodic boundary conditions for DG

[Steven Vandekerckhove]

Several QA notes can be found on the forum [1, 2, 3], but since I can't find a trace of this issue, I decided to add it here to keep it in mind.

[1] http://fenicsproject.org/qa/6779/should-periodic-boundary-conditions-function-space-ghosted?show=6779#q6779 [2] http://fenicsproject.org/qa/3119/where-to-find-what-has-been-resolved-in-fenics?show=3119#q3119 [3] http://fenicsproject.org/qa/458/dg-periodic-boundary-conditions?show=459#a459

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[Prof Garth Wells]

@cwilson and @chris_richardson are working on this.