Feature: Test for Poisson equation with periodic boundary conditions and test for adaptivity

This branch is Linus' work imported from https://github.com/LSenf/dune-pdelab.git, with a minor post-cleanup.

It provides various tests for the Poisson equation with periodic boundary conditions:

• 2D case with example taken from http://fenicsproject.org/documentation/dolfin/dev/python/demo/documented/periodic/python/documentation.html
  • using a Q1-CG discretization
  • using a Q2-CG discretization
  • using a Q1-DG discretization
  • using a Q2-DG discretization
• the example from http://fenicsproject.org/documentation/dolfin/dev/python/demo/documented/periodic/python/documentation.html trivially extended to 3D with periodic boundary conditions in x- and y-direction
  • using a Q1-CG discretization
  • using a Q2-CG discretization
  • using a Q1-DG discretization
  • using a Q2-DG discretization

The tests concerning adaptivity have been revised.