Have filename suffix when exporting the fdmat-obj. and the raw data

[bpollner]

Like in the plotgates function, it would be nice to have the possibility to add a custom filename suffix to the name when exporting (also automatically) the fdmat-object and the file containing the raw data.
Because so far, these names are always the same when using flowdexit, and the user has to manually change / modify the name so that previous files do not get overwritten.