Deaths time conversion factor

[daniel-klein]

Deaths has:

if isinstance(death_rate, ss.TimePar):
    factor = self.t.dt # TODO: figure out why this isn't 1.0
else:
    factor = ss.time_ratio(unit1=self.t.unit, dt1=self.t.dt, unit2='year', dt2=1.0)

But this appears to be an error. The factor should be 1.0 as in Births if using unit-away rates, which is on this codepath.

[cliffckerr]

Should add a test asserting birth and death rates