use mocking for cpu count during tests

test/test_run_modules/test_pre_processing.py

 import logging
 
 import pytest
+import mock
 
 from mlair.data_handler import DefaultDataHandler, DataCollection, AbstractDataHandler
 from mlair.helpers.datastore import NameNotFoundInScope
@@ -34,7 +35,8 @@ class TestPreProcessing:
         yield pre
         RunEnvironment().__del__()
 
-    def test_init(self, caplog):
+    @mock.patch("multiprocessing.cpu_count", return_value=1)
+    def test_init(self, mock_cpu, caplog):
         ExperimentSetup(stations=['DEBW107', 'DEBY081', 'DEBW013', 'DEBW076', 'DEBW087'],
                         statistics_per_var={'o3': 'dma8eu', 'temp': 'maximum'})
         caplog.clear()