Introduce ShapeWarning

Author: michaelosthege

pymc3/exceptions.py

@@ -17,6 +17,7 @@
     "IncorrectArgumentsError",
     "TraceDirectoryError",
     "ImputationWarning",
+    "ShapeWarning",
     "ShapeError",
 ]
 
@@ -41,6 +42,12 @@ class ImputationWarning(UserWarning):
     pass
 
 
+class ShapeWarning(UserWarning):
+    """ Something that could lead to shape problems down the line. """
+
+    pass
+
+
 class ShapeError(Exception):
     """Error that the shape of a variable is incorrect."""