use 16 kernels in maximum for parallel preprocessing

mlair/run_modules/pre_processing.py

@@ -265,7 +265,8 @@ class PreProcessing(RunEnvironment):
 
         if multiprocessing.cpu_count() > 1:  # parallel solution
             logging.info("use parallel validate station approach")
-            pool = multiprocessing.Pool(psutil.cpu_count(logical=False))  # use only physical cpus
+            pool = multiprocessing.Pool(
+                min([psutil.cpu_count(logical=False), len(set_stations), 16]))  # use only physical cpus
             logging.info(f"running {getattr(pool, '_processes')} processes in parallel")
             output = [
                 pool.apply_async(f_proc, args=(data_handler, station, set_name, store_processed_data), kwds=kwargs)