Moved utility templates into dtl namespace
* RelativePointer also resides in dtl as it shouldn't be exposed to the library user
** altough helper templates such as enable_if are useful outside BinaryMapping it is not a goal of this library to provide these templates
** possibility: extract general helper templates into separate library to be used in different projects
* Why _dtl_ and not _detail_?
** detail was to verbose for writing it in front of all identifiers, dtl is as short as std
* Modified all users of the affected templates accordingly