Implementing the EOC test.

TODO

examples/heat-equation/tnl-run-heat-equation-eoc-test

src/functions/tnlTestFunction.h

src/implementation/CMakeLists.txt

src/implementation/functions/CMakeLists.txt

src/implementation/functions/tnlTestFunction_impl.cpp

src/implementation/functions/tnlTestFunction_impl.cu

src/implementation/functions/tnlTestFunction_impl.h

...mentation/operators/tnlDirichletBoundaryConditions_impl.h

src/operators/tnlDirichletBoundaryConditions.h

tools/src/tnl-diff.cpp

tools/src/tnl-diff.h