Rare failing assert in RTI line 3290

[joehcollins]

Noted that when running the cohort model on myriad i'm getting a rare error crashing a small number of runs at line 3290 of RTI:

assert date_values[date_column] >= self.sim.date This fails when someones date of recovery is 'in the past'

I will create a PR replacing the assert with an error and add in a test - hopefully can work out the issue