[bugfix,interpolate] fix #83 and allow interpolation from generic lambdas

we used Dune::Functions::IsCallable, which is part of the SignatureTraits to check if we are dealing with a callable. This construct only works if we have a unique operator() (like the while signature traits can only work in this case). There is an alternative check as part of the dune-functions concepts Dune::Functions::Concept::isCallable<F,Domain>. It requires that we know the parameters Domain and then it tests if F(Domain) is callable. As we can deduce Domain from the GV passed to makeLocalFunctionTree, we can now interpolate from a generic lambda.