added pygrib to dependencies for mch tests

Author: wolfidan

.github/workflows/pyrad_tests_mch.yml

@@ -1,7 +1,7 @@
 name: Test pyrad mch
 env:
   MINIMAL_DEPENDENCIES: Cython numpy cartopy
-  TEST_DEPENDENCIES: pytest imageio gdal==3.5.3
+  TEST_DEPENDENCIES: pytest imageio pygrib gdal==3.5.3
 
 on:
   # Triggers the workflow on push or pull request events but only for the master branch