provide export branch for libstfinv
libstfinv ist used in DENISE for example. When libstfinv and DENISE lived in their subversion-repositories previously, export branches in TFSoftware and vendor import-branches in DENISE existed in order to update library versions (libstfinv, libfourier, libaff, libcseife) in DENISE. These branches at least partly got lost during migration to git. They shall be recreated.