[disc] Introduce discretization tags in namespace DiscretizationMethods

Tags have some advantages over enums. There are separate types and allow
for things like tag dispatch. They are also extensible. That means you
can now define your own discretization tag in a downstream module and
specialize classes for it. With enums you always need to modify the dumux core,
i.e. add the enum to the list of discretization method in this header.