Clean up

pystencils/config.py

-import warnings
 from copy import copy
 from collections import defaultdict
 from dataclasses import dataclass, field
@@ -147,6 +146,9 @@ class CreateKernelConfig:
                 self.backend = Backend.CUDA
             else:
                 raise NotImplementedError(f'Target {self.target} has no default backend')
+            
+        if not isinstance(self.backend, Backend):
+            raise ValueError("backend must be provided by the 'Backend' enum")
 
         #  Normalise data types
         if not isinstance(self.data_type, dict):