Improving the Dirichlet boundary conditions.

TODO

examples/heat-equation/heatEquationSolver_impl.h

examples/heat-equation/tnl-heat-equation-eoc.cpp

examples/heat-equation/tnl-heat-equation.cpp

examples/heat-equation/tnl-run-heat-equation

examples/heat-equation/tnlHeatEquationEocRhs.h

src/functions/tnlConstantFunction.h

src/functions/tnlTestFunction.h

src/implementation/functions/tnlConstantFunction_impl.h

src/implementation/functions/tnlTestFunction_impl.h

src/implementation/mesh/tnlTraversal_Grid3D_impl.h

src/implementation/operators/CMakeLists.txt

...n/operators/tnlAnalyticDirichletBoundaryConditions_impl.h

...mentation/operators/tnlDirichletBoundaryConditions_impl.h

src/operators/CMakeLists.txt

src/operators/tnlAnalyticDirichletBoundaryConditions.h

src/operators/tnlDirichletBoundaryConditions.h

...unit-tests/operators/diffusion/tnlLinearDiffusionTest.cpp

tests/unit-tests/operators/tnlPDEOperatorEocTester.h

tests/unit-tests/tnlApproximationError.h