Require linear parameter for Arkane statmech jobs

Previously, no error was raised when the `linear` parameter was not
implemented properly. This could hide information about incorrect
calculations if the user is not properly implementing the parameter.

This commit raises an error if no `linear` parameter is given.

arkane/statmech.py

@@ -254,7 +254,8 @@ def load(self, pdep=False):
         try:
             linear = local_context['linear']
         except KeyError:
-            externalSymmetry = None
+            logging.error('You did not set whether the molecule is linear with the required `linear` parameter')
+            raise
 
         try:
             externalSymmetry = local_context['externalSymmetry']