* added phasefield process and test.

* apply clang-format.

* added missing integralMeasure for axisymmetric problem.

* fixed compilation errors after rebasing.

* added unconfined compression ctest.

* added missing reference.

* renamed specialFunction as calculateDegradedStress.

* changed order of stress and strain components.

* deleted unused file.

* changed IntegrationPointData.

* added local results to highlight the deviation.

* returned DeltaXCheck true.

* modified dimension in ProjectData.

* removed second order mesh.

* moved history_variable to pushBackState.

* added absolute error of displacement for beam3d test.

* removed eval() calls in calculation of stress components.

* added description for the calculateDegradedStress function.

* added types available.

* rephrased description.

* removed unused C variable.

* added model description.

* modified types of constitutive relation.

* removed CreateLinearElasticPhaseField.h.

* added a tag for phasefield_parameters.

* added missing phasefield_parameters after PHASE_FIELD.

* removed eval() calls in calculation of tensile stress component.

* moved process description to PhaseFieldProcess.h.

* added description to the undocumented function.

* created missing file in documentation.

* removed two duplicated files in phasefield process.

* removed unused file in documentation.

* removed the call to material_model.integrateStress.

* removed unused variables.

* fixed expression of length scale parameter in documentation.