Set votes_type in Iowa loader

Set the votes_type RawResult attribute in the Iowa loader for
preprocessed results.

Update methods for determining votes_type to only worry about
absentee and provisional votes rather than over/under votes.
Over/under votes are more like pseduo-candidates.  In any case,
they're handled in transforms.

Update tests to reflect these changes.

Addresses https://github.com/openelections/core/issues/203