replaced stringstream by C++11 to_string()

Summary:
Signed-off-by: Clemens Hentschke <c.hentschke@dkfz-heidelberg.de>

Resolved T21650: removed unneeded classes, structs and typedefs in rttbBase.h

Signed-off-by: Clemens Hentschke <c.hentschke@dkfz-heidelberg.de>

Merge branch 'T21650-overheadBaseType'

Test Plan: tests run fine

Reviewers: strubelp

Reviewed By: strubelp

Maniphest Tasks: T21650

Differential Revision: https://phabricator.mitk.org/D54