Add predicted turnout to unit data

[lennybronner]

Description

If we are running a margin model we need to save the turnout predictions also to be able to evaluate the model.

Jira Ticket

Test Steps

tox
also

elexmodel 2017-11-07_VA_G --estimands=margin --office_id=G --geographic_unit_type=precinct --pi_method=bootstrap --percent_reporting=10 --aggregates=postal_code --fixed_effects=county_classification --features=baseline_normalized_margin --aggregates=unit

Now includes pred_turnout for the unit_data