Add utilities to work with integer_sequence

Summary

When working with generic algorithms of flexible dimensions, e.g. with arbitrary rank tensors, one often needs integer sequences as a utility. The reason often is, that there are no variadic pack utilities yet in the language and one has to create these packs using integer sequences. This MR provides some utilities to work with ineteger sequences similar as with containers, e.g., access specific entries, push front/back indices, sort or make set difference operations...

Application

These utilities are extracted from a work in the dune-tensor module: simon.praetorius/dune-tensor!71

ToDo

• Some utilities exist already but are spread over several files, e.g. unpackIntegerSequence, integerSequenceEntry. Mabye could be collected or aliased.