Get rid of diffusion* local operators

We want to settle on a single implementation, so just use the more generic convectiondiffusion* versions instead. The existing ones should be deprecated (classes and files) and tests and examples in the howto ported to the new operators.

The only problem is diffusionmixed.hh, we don't have an equivalent for that right now. So unless someone is extra-eager and wants to write an convectiondiffusionmixed.hh, that file will have to stay. In that case, it needs to be cleaned up w.r.t. entity interface and quadrature loops.