Merge branch 'feature/compare_solution_to_dirichlet_bc' into 'master'

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