checkout file for specific version

This should fix #405 

If a version of an ontology was sent to Hets, it was checked out from the git repository to a file.
When sending the ontology again to hets, the version is only checked out, if the (previously created) file doesn't exist any more.
The filename didn't change with a new version though.
This way, the old version is sent to hets.

The fix creates distinct files for each version.