Don't complain if target directory exists, but empty

Since the last commit already creates the directory (to make sure
copying the HDF5 file works), shutil.copytree bailed without the
`dirs_exist_ok` flag.  If the directory is not empty we raise an error
though.

pyiron_base/job/core.py

@@ -721,9 +721,12 @@ def _internal_copy_to(self, project=None, new_job_name=None, new_database_entry=
 
         # Copy files outside the HDF5 file
         if copy_files and os.path.exists(self.working_directory):
+            if len(os.listdir(self.working_directory)) > 0:
+                raise RuntimeError("Target directory for copy not empty!")
             shutil.copytree(
                 self.working_directory,
                 new_job_core.working_directory,
+                dirs_exist_ok=True
             )
         return new_job_core, file_project, hdf5_project, False