Use calc's evaluator for staff answers in `Numercial` type

Rather than just calling `complex(...)` on the staff answers, also call
`calc.evaluator`. This way, simple things like `1/3` can be entered, as well
as more complicated explicit equations (e.g. `sqrt((12^2 + 26^2)/2)` etc, etc)

Simplify and update the tests to work with this.
Merge pull request #726 from edx/peterb/numerical/evaluate-staff

Use calc's evaluator for staff answers in `Numercial` type