Fixing simple heat equation benchmark.

src/TNL/Devices/Cuda.cpp

src/TNL/Devices/Cuda.h

src/TNL/Operators/diffusion/LinearDiffusion_impl.h

src/TNL/Problems/HeatEquationProblem_impl.h

src/TNL/Solvers/SolverStarter_impl.h

src/TNL/Timer.cpp

src/TNL/Timer.h

...at-equation-benchmark/HeatEquationBenchmarkProblem_impl.h

tests/benchmarks/heat-equation-benchmark/pure-c-rhs.h

...t-equation-benchmark/tnl-benchmark-simple-heat-equation.h