Move get_reference_tree to Tree class

src/cladetime/cladetime.py

@@ -194,17 +194,3 @@ def _validate_as_of_date(self, as_of: str) -> datetime:
             raise CladeTimeInvalidDateError(f"Date must be after May 1, 2023: {as_of_date}")
 
         return as_of_date
-
-    def get_reference_tree(self) -> dict:
-        """Return a reference tree used for SARS-CoV-2 clade assignments
-
-        Retrieves the reference tree that was current as of
-        :any:`tree_as_of<tree_as_of>`.
-
-        This method is not yet implemented.
-
-        Returns
-        -------
-        dict
-        """
-        return {self.tree_as_of: "not implemented"}

src/cladetime/util/tree.py

@@ -108,3 +108,17 @@ def _get_url_ncov_metadata(self):
             self._clade_time._config.nextstrain_ncov_metadata_key,
             self.as_of,
         )[1]
+
+    def get_reference_tree(self) -> dict:
+        """Return a reference tree used for SARS-CoV-2 clade assignments
+
+        Retrieves the reference tree that was current as of
+        :any:`tree_as_of<tree_as_of>`.
+
+        This method is not yet implemented.
+
+        Returns
+        -------
+        dict
+        """
+        return {self.tree_as_of: "not implemented"}