\noindent{\huge \textbf{Declaration}}\\\\\\ I hereby declare that this thesis has been composed by me without any assistance and I have not used any sources or tools other than those cited. Furthermore, I declare that I have acknowledged the work of others by providing detailed references. I also declare that this thesis or parts of it have not been accepted in any other previous application for a degree, project or examination. \vfill \rule{5cm}{.5pt}\\ \phantom{ab}Freiburg, \today