Make frozendict picklable (#69)

* Replace frozendict implementation by existing stdlib thing
  which behaves more or less the same, but coops with pickle

src/anemoi/inference/metadata.py

@@ -13,6 +13,7 @@
 import warnings
 from collections import defaultdict
 from functools import cached_property
+from types import MappingProxyType as frozendict
 from typing import Literal
 
 import numpy as np
@@ -37,15 +38,6 @@ def _remove_full_paths(x):
     return x
 
 
-class frozendict(dict):
-    def __deepcopy__(self, memo):
-        # As this is a frozendict, we can return the same object
-        return self
-
-    def __setitem__(self, key, value):
-        raise TypeError("frozendict is immutable")
-
-
 class Metadata(PatchMixin, LegacyMixin):
     """An object that holds metadata of a checkpoint."""