Small refactoring of the code.

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

examples/heat-equation/tnl-heat-equation.h

src/core/cuda/cuda-reduction_impl.h

src/problems/tnlHeatEquationProblem.h

src/problems/tnlHeatEquationProblem_impl.h

src/problems/tnlPDEProblem.h

src/solvers/CMakeLists.txt

src/solvers/pde/tnlExplicitTimeStepper.h

src/solvers/pde/tnlExplicitTimeStepper_impl.h

src/solvers/pde/tnlPDESolver.h

src/solvers/pde/tnlPDESolver_impl.h

src/solvers/pde/tnlSemiImplicitTimeStepper.h

src/solvers/pde/tnlSemiImplicitTimeStepper_impl.h

src/solvers/tnlBuildConfigTags.h

src/solvers/tnlFastBuildConfigTag.h

src/solvers/tnlSolver.h

src/solvers/tnlSolverConfig_impl.h

src/solvers/tnlSolverInitiator.h

src/solvers/tnlSolverInitiator_impl.h

src/solvers/tnlSolverStarter_impl.h