[cleanup] Normalize single-inclusion macro
Remove unused default implementation of method IndexSet::subIndex

It just consisted of a DUNE_THROW(NotImplemented).  What good is a default
implementation that throws NotImplemented?
[cleanup] Remove an unused remove_const

The Traits type already has its const stripped at the beginning of the class.
[bugfix,documentation] Fix spelling
[cleanup] Remove two unused headers
[cleanup] EntityType --> Entity for more consistency