Implemeting heat equation benchmark.

Conflicts:
	CMakeLists.txt
	src/core/vectors/tnlVector_impl.h
	tests/benchmarks/tnl-cuda-benchmarks.h
	tests/unit-tests/operators/diffusion/CMakeLists.txt

Conflicts:
	src/functions/tnlOperatorFunction.h
	src/operators/tnlFunctionInverseOperator.h
	src/operators/tnlOperator.h
	src/operators/tnlOperatorComposition.h
	tests/unit-tests/operators/diffusion/CMakeLists.txt

Implementing operator function test.