[dune-pdelab] ImplementationsTag for different grid functions

[Christian Engwer]

Hi Steffen,

why do we use different ImplementationTags for composite-, power- and
"plain" gridfunctions?

As fas as I understood the tags, the ImplementationTag describes a
concept of functionality, while the NodeTag specifies the type of node
in the tree. Thus I'd expect that we only need GridFunctionTag for all
node type, as they differ in the definition of NodeTag?! Did I miss
something? At least I'd expect that CompositeGridFunctionTag and
PowerGridFunctionTag inherit GridFunctionTag.

Ciao
Christian