move internal stuff to sub-directory detail

[Peter]

as usual namespaces and filestructure should mirror each other to place internal stuff in sub-namespace detail.

Perhaps, to make it even clearer it is internal stuff, use internal rather than detail.

[Peter]

(In [1591]) fixes #447

Moved some (internal) functions and clases to namespace theplu::yat::utility::detail. Did not move the stuff to another file though, because unexpected problems with cyclic dependency.

Currently the namespace theplu::yat::utility::detail is not documented (doc/namespace.doxygen), which implies that functionality is excluded from documentation.