implement unit test for tixiRenameElement
implement unit test for tixiRenameElement

addresses #113

melven/tixi,melven/tixi,DLR-SC/tixi,melven/tixi,melven/tixi,DLR-SC/tixi,DLR-SC/tixi,DLR-SC/tixi,DLR-SC/tixi,melven/tixi