Add security routine for output file import

When running simulations in parallel during calibration routines with the package mlrMBO it might occur that the API tries to read in files before they are written out completely. The same problem occurred for rogs5.

I suggest putting a check at base-level (e.g. in the function ogs6_read_output_files), that checks if the last thread writing the file out has finished. Or simply checking if the file exists, and is not empty in a while loop with timeout...