Make OutputAttachable methods return whether or not should the processing continue.
Fixes in NewtonSolver.
Additions to Newton:

- NewtonException
- vector storing residual and solution norms (using Output Attachable mixin)
- Add NewtonSolverConvergenceMeasurement enum &  newtonConverged global function (this should probably be improved).
Gcc fixes.
Rework NewtonSolver.
Fix hcurl example.
Add nonlinearity test example.
Testing & fixes.
Only add compiler flags for OpenMP for MSVC.
Add on_initial_step_end().
Work on Adaptivity.
Progress on unification of adaptivity mechanisms with the rest of the code.
More adaptivity-related progress.
Add 01-intro.
Progress on adaptivity.
Progress on Adapt.
Fixes in test examples.
Finish with the adaptivity reorganization for now.
Redo test examples.
Revert Newton according to devel.