Removing parameter mesh from method getValue in operators.

The condition substituting nextIteration() is different than in the
method itself, because incrementing the iteration counter is avoided.

Refactoring the Dirichlet and the Neumann boundary conditions.

...and some simplifications in tnlSolverStarter - needs more!

Adding simple heat equation benchmark for CUDA.

The system is actually solved for t+tau, not t.

The nextIteration() method should not be called here because it
increments the iteration counter.