Replace FitVal with UFloat

[nkanazawa1989]

What is the expected behavior?

With #551 we replace FitVal object with UFloat so that we can directly compute error propagation while keeping the parameter correlation. However, once we save the result in the result db, the UFloat is typecasted to FitVal and we cannot compute error propagation any more. We should update database service (json hook) to directly store the UFloat object.