Changed declaration to be consistent with TU "Erklaerung"

(Commissioned by CR) Declaration is now consistent with https://www.tu-darmstadt.de/media/dezernat_ii/referat_iig/formulare_vorlagen/pm_1/erklaerungen/Erklaerung_zur_Abschlussarbeit_Vorlage-fuer_digitale_Abgabe.docx

For this \makedeclaration[multi]{Autor 1, Autor 2} gets an argument "multi" or "single" for multiple or single authors. Setting "single" will change pronouns to "ich" and add the sentence "Abschlussarbeiten dürfen nur einmal wiederholt werden." to be indentical to the TU Erklaerung (which is necessary for Abschlussarbeiten). If it is a single author not writing a Abschlussarbeit this sentence is still inserted, which I think is no problem.