update nduncertainty for multiple/divide

While trying out the nddata and nduncertainity classes, I noticed that the propagation of errors for multiple/divide did not work.   The problem was that in propagate_divide and propagate_multiply they were calling a self.data that didn't exist.   To fix this problem, I changed self.data to self.parent_nddata.data in those two tasks.  I also added 'self._parent_nddata=value' to the setter function for parent_nddata.   

In addition, I added two tests to test_nddata to test for propogation of errors when using multiply and divide.