Optional event_col in AAF and CPH fitters

@spacecowboy, this PR adds the option for the `event_col` parameter (default None). If left as `None`, then we assume all non-censored (similar to the kmf fitter). 

Also, `duration_col` is now a mandatory argument. 
df[None] = 1 ? 

Shouldn't we first invent a name for the event column? e.g.

    if event_col is None:
        event_col = 'event_col'
       df[event_col] = 1