Utilize parallelization for the maskregion operation

SEPTREFHUM.py

@@ -180,7 +180,7 @@ subprocess.run(f'cdo -s -aexpr,"WARNING_STATUS = WHS <= 0 ? 0 : -1; WARNING_STAT
 if os.getenv("MASK_FILE") is not None:
     mask_file = os.getenv("MASK_FILE")
     print(f"Applying mask file {mask_file} to result.nc")
-    subprocess.run(f'cdo -maskregion,{mask_file} {tmpfile_path}result_unmasked.nc {tmpfile_path}result.nc', shell=True)
+    subprocess.run(f'cdo -P 6 -maskregion,{mask_file} {tmpfile_path}result_unmasked.nc {tmpfile_path}result.nc', shell=True)
 else:
     os.rename(f"{tmpfile_path}result_unmasked.nc", f"{tmpfile_path}result.nc")
 #"""