Add warning when accessing new molecules data

mp_api/client/mprester.py

@@ -293,6 +293,15 @@ def __exit__(self, exc_type, exc_val, exc_tb):
         """Support for "with" context."""
         self.session.close()
 
+    def __getattribute__(self, attr):
+        if "molecules" in attr:
+            warnings.warn(
+                "NOTE: You are accessing a new set of molecules data to be officially released very soon. "
+                "This dataset includes many new properties, and is designed to be more easily expanded. "
+                "For the previous (legacy) molecules data, use the MPRester.legacy_jcesr rester. "
+            )
+        return super().__getattribute__(attr)
+
     def __getattr__(self, attr):
         if attr == "alloys":
             raise MPRestError(