Change the install path for the doc
Change the install path for the doc

This commit changes the install path for the documentation. Previously: install/share/doc/tixi. Now: install/share/tixi3/doc

addresses #87

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