Refactoring the heat equation.

examples/heat-equation/heatEquationSetter.h

examples/heat-equation/heatEquationSetter_impl.h

examples/heat-equation/heatEquationSolver_impl.h

examples/heat-equation/tnlDirichletBoundaryConditions.h

examples/heat-equation/tnlDirichletBoundaryConditions_impl.h

src/functions/CMakeLists.txt

src/functions/tnlConstantFunction.h

src/functions/tnlExpBumpFunction.h

src/functions/tnlFunctionDiscretizer.h

src/functions/tnlSinBumpsFunction.h

src/functions/tnlSinWaveFunction.h

src/functions/tnlTestFunction.h

src/implementation/functions/CMakeLists.txt

src/implementation/functions/tnlConstantFunction_impl.h

src/implementation/functions/tnlExpBumpFunction_impl.h

src/implementation/functions/tnlSinBumpsFunction_impl.h

src/implementation/functions/tnlSinWaveFunction_impl.h

src/implementation/mesh/tnlGrid1D_impl.h

src/implementation/mesh/tnlGrid3D_impl.h

tools/src/tnl-init.h