Modify TypeError to ValueError for getValue() when op._value = None

[bobleesj]

The full description is provided in the following PR requests: #71

[ycexiao]

@sbillinge, I still think we don't need to change the TypeError.

op._value=None is not the actual cause for TypeError. The actual cause is that the operator requires two inputs, but there is none given.

-> self._value = self.operation(*vals)
(Pdb) n
TypeError: add() takes from 2 to 3 positional arguments but 0 were given

So the problem is with the function add, and we don't need to wrap the TypeError in ValueError in this case.

[sbillinge]

@ycexiao I am moving this to a later milestone to not hold things up. I agree that the code is throwing the right exception. I think that Pavol's point was probably that it would be more helpful to the user to know that the error arose because of an invalid value to an attribute, so we may want to modify the way the code works so that it throws a ValueError under that circumstance. It is a bit hard to fix without knowing what Pavol's actual intent was though.