Merge pull request #2145 from TomFischer/ConstraintBCs

...undaryCondition/c_ConstraintDirichletBoundaryCondition.md

...chletBoundaryCondition/t_constraining_process_variable.md

...raintDirichletBoundaryCondition/t_constraint_direction.md

...raintDirichletBoundaryCondition/t_constraint_threshold.md

...ConstraintDirichletBoundaryCondition/t_constraint_type.md

...ition/ConstraintDirichletBoundaryCondition/t_parameter.md

MeshLib/Elements/Utils.h

ProcessLib/BoundaryCondition/BoundaryCondition.cpp

ProcessLib/BoundaryCondition/BoundaryCondition.h

ProcessLib/BoundaryCondition/BoundaryConditionCollection.cpp

ProcessLib/BoundaryCondition/BoundaryConditionCollection.h

...oundaryCondition/ConstraintDirichletBoundaryCondition.cpp

.../BoundaryCondition/ConstraintDirichletBoundaryCondition.h

...tion/ConstraintDirichletBoundaryConditionLocalAssembler.h

ProcessLib/HT/Tests.cmake

ProcessLib/Process.cpp

ProcessLib/ProcessVariable.cpp

ProcessLib/ProcessVariable.h

...ConstViscosityThermalConvection_pcs_0_ts_0_t_0.000000.vtu

...ConstViscosityThermalConvection_pcs_0_ts_1_t_0.000000.vtu