Deprecate vec_access(), vec_size() in favor of operator[]() and size()

The FieldVector and DynamicVector classes in DUNE provide the methods vec_access() and vec_size() which behave like operator[]() (size()). Their only purpose seems to allow the DenseVector facade class to forward calls to operator[]() and size() to asImp().vec_access() (asImp().vec_size()), however it could just call asImp()[] (asImp().size()) directly.

I suggest to deprecate (or directly remove?) these methods.

Note that these methods don't seem to be used elsewhere. I grepped all core modules for vec_access and vec_access and found them only used in dune-common. (In particular dune-istl's BlockVector doesn't seem to provide them.)