Implemeting heat equation benchmark.

Moved tnlObject_impl.h back to tnlObject.cpp since function definitions
in header files cause duplicates when building libtnl. The constructor
of tnlObject is __cuda_callable__ so it's empty definition was moved
directly to tnlObject.h to make it visible to nvcc.

Problem with marking tnlObject's virtual destructor as __cuda_callable__
is yet to be solved...

Adding explicit template instantiation.
Moving some _impl.h files into implementation dir.

Finishing new template interface for GMRES.

Adding unit tests for tnlString, tnlObject and tnlArrayManager.

There is gap of missing commits.

Adding edge-skip to grid comparison function.