Compare DBC to numerical solution

Makes it possible to compare numerical solution and dirichlet boundary condition by setting the formcompiler option '--dirichlet-is-exact-solution'.

Comparing means finding the maximum error in the dof vectors and throwing an error if it is too large. If you have any suggestions for better comparisons or option/functions names just let me know. 

See merge request !5

Solve nonlinear problem

Make it possible to solve simple nonlinear problems.

It would be nice to have a DG test case but we don't have a lop for creating a reference solution. I will open an issue for that and remove WIP from this merge request.

See merge request !4

Merge branch 'feature/nonlinear' of http://conan2.iwr.uni-heidelberg.de/git/dominic/dune-perftool into feature/nonlinear

Merge branch 'feature/nonlinear' of http://conan2.iwr.uni-heidelberg.de/git/dominic/dune-perftool into feature/nonlinear

Use loopy for evaluation of basis gradient

Implements #17

See merge request !2

Add include to vtkpredicate

Needed  to build all the tests.

See merge request !1

including CallInstructions for accumulation etc.

major remodelling of the DuneTarget

targetting better indexing logic.

because any non-committed changes get lost upon reconfiguration.