Feature/remove indexpack

std::index_sequence can be used as a replacement for index_pack. This MR removes index_pack and the various construction mechanisms.

Strictly speaking, this is a breaking interface change since the classes are removed without deprecation. However, they are helper classes for experts which should have replaced their usage long ago. E.g. they are (now) no longer used in dune-typetree and dune-pdelab at all. In view of their std:: analouges we better remove them straight ahead to stop advertising them by there mere presence.