Implemented const-correctness through the whole stack
* moved main Tuple template implementation into dtl namespace
** BinaryMapping::Tuple is now just a helper template to hide dtl::Tuple implementation details
*** Tuple offers the user the possibility of specializing her tuple the same way as before this commit
*** ... while offering the Container template the possibility of specializing the base type
*** this was needed to support Tuple template specializations with const base pointer types
* changed Mapper template and its Modifier templates to be specializable on a base type
** this was needed to make construction of Tuple instances from const base pointers possible
* added base type argument to BufferIterator template and changed Buffer template implementation accordingly
* changed Container and Iterator implementations to match these changes
* => the BinaryMapping implementation should now be completely const-correct
* remaining problem: value_type consists of const types when Tuple's types are const
** this is not desirable as we want to be able to change value_type instances as they don't change the source memory
* ToDo: document requirements on Container specialization types
** they have expanded substantially through the changes of this commit