Enable multiplying NDCube and NDData

ndcube/ndcube.py

@@ -963,7 +963,7 @@
     def __neg__(self):
         return self._new_instance(data=-self.data)
 
-    def add(self, value, handle_mask=np.logical_and):
+    def _operate_arithmetic(self, operation, value, handle_mask=np.logical_and):
         """
         Users are allowed to choose whether they want handle_mask to be AND / OR .
         """
@@ -1016,6 +1016,9 @@
         # return the new NDCube instance
         return self._new_instance(**kwargs)
 
+    def add(self, operation, value, handle_mask = np.logical_and):
+        return self._operate_arithmetic(operation, value, handle_mask)
+
     def __add__(self, value):
         # when value has a mask, raise error and point user to the add method. TODO
         #
@@ -1055,6 +1058,9 @@
     def __rsub__(self, value):
         return self.__neg__().__add__(value)
 
+    def multiply(self, operation, value, handle_mask = np.logical_and):
+        return self._operate_arithmetic(operation, value, handle_mask)
+
     def __mul__(self, value):
         if hasattr(value, 'unit'):
             if isinstance(value, u.Quantity):