delete tmp directory by default
gather_variables.sh: Empty tmpdir by default before running the script but ask for user confirmation.
This is necessary because the merging process will use all files in the tmpdir that have the defined set of variables. This might include previously processed ensemble members that are not selected in the script. The previously created files might even contain errors, which will cause the script to crash even though the selected ensemble members are all ok.
An alternative approach would be to make the selection of files in the ensemble merging process more restrictive, for example by creating a file list in the script during runtime and only merging those files in the end.