ENH: Add CaretMetaData._sanitize static method to focus warning code

Author: effigies

nibabel/caret.py

@@ -45,8 +45,15 @@ class CaretMetaData(xml.XmlSerializable, MutableMapping):
     b'<MetaData><MD><Name>key</Name><Value>val</Value></MD></MetaData>'
     """
     def __init__(self, *args, **kwargs):
+        args, kwargs = self._sanitize(args, kwargs)
         self._data = dict(*args, **kwargs)
 
+    @staticmethod
+    def _sanitize(args, kwargs):
+        """ Override in subclasses to accept and warn on previous invocations
+        """
+        return args, kwargs
+
     def __getitem__(self, key):
         """ Get metadata entry by name