Add ComposedGridFunction

For given inner grid functions g0, ..., gn and an outer function f this implements the grid function mapping x to f(g0(x), ..., gn(x)). It is assumed that the grid functions are defined on the same EntitySet and that their range type can be passed to the outer function.

All functions are stored by value. Storing references is possible using passing std::ref(f). To resolve extended methods internally this makes use of the Dune::Functions::resolveRef() utility.

This is based on (i.e. includes) !315 (merged).