Replace integerSequenceEntry by get

The utility Dune::integerSequenceEntry and Dune::IntegerSequenceEntry are outdated and should be replaced by Dune::get(std::integer_sequence<...>, index) or Dune::get<index>(std::integer_sequence<...>). I have found only one location where it is used. This is fixed already. Note that the class Dune::integerSequenceEntry is not yet properly replaced, but the Dune::get utility from integersequence.hh would also return an std::integral_constant and thus could be used as replacement.

The old implementation integerSequenceEntry was by recursion over the index. This could be very time consuming, but on the other hand it could be statically cached by the compiler. So, it is not clear whether this would give a compile-time performance improvement, but at least, the instantiation depth is drastically reduced by the implementation of Dune::get, which uses an array internally.