Add resolving string to NestedChainMap

This is in preparation for solving AstarVienna/ScopeSim#387

A resolving string is a string key ending in `"!"`. An instance of `NestedChainMap` will only continue to follow references if the key is a resolving key, otherwise it will return the value a the bang-string that is stored.

astar_utils/nested_mapping.py

@@ -276,7 +276,7 @@ class NestedChainMap(ChainMap):
 
     def __getitem__(self, key):
         """x.__getitem__(y) <==> x[y]."""
-        value = super().__getitem__(key)
+        value = super().__getitem__(key.removesuffix("!"))
 
         if isinstance(value, abc.Mapping):
             submaps = tuple(RecursiveNestedMapping.from_maps(self.maps, key))
@@ -285,7 +285,7 @@ def __getitem__(self, key):
                 return submaps[0]
             return NestedChainMap(*submaps)
 
-        if is_bangkey(value):
+        if is_bangkey(value) and is_resolving_key(key):
             value = self[value]
         return value
 
@@ -306,6 +306,11 @@ def is_bangkey(key) -> bool:
     return isinstance(key, str) and key.startswith("!")
 
 
+def is_resolving_key(key) -> bool:
+    """Return ``True`` if the key is a ``str`` and ends with a "!"."""
+    return isinstance(key, str) and key.endswith("!")
+
+
 def is_nested_mapping(mapping) -> bool:
     """Return ``True`` if `mapping` contains any further map as a value."""
     if not isinstance(mapping, abc.Mapping):