Merge branch 'bug-is-negative' into 'master'

Bugfix: TypedSymbol.is_negative should not be implemented in terms of super().is_positive

See merge request !31

pystencils/data_types.py

@@ -141,7 +141,7 @@ class TypedSymbol(sp.Symbol):
             if np.issubdtype(self.dtype.numpy_dtype, np.unsignedinteger):
                 return False
 
-        return super().is_positive
+        return super().is_negative
 
     @property
     def is_nonnegative(self):