[Typetraits] Follow dune-common transition from SelectType to C++11-like conditional
[Headercheck] Don't include function.hh from typetraits.hh to avoid an invalid circular inclusion

function.hh includes TypeTree headers which in turn require typetraits.hh for
decltpr<>(), so let's not do that. Apart from that, having typetraits.hh depend
on the much higher-level function.hh smells like bad layering...
Merge branch 'work'

Just some small fixes

* work:
  [Headercheck] Don't include function.hh from typetraits.hh to avoid an invalid circular inclusion
  [Typetraits] Follow dune-common transition from SelectType to C++11-like conditional