Figure out date usage

[robynstuart]

@MObrien-IDM or @mzimmermann-IDM can you clarify how the lists of dates are used? These ones:

    # Dates
    State('dobs',               [], list),  # Dates of birth of children
    State('still_dates',        [], list),  # Dates of stillbirths -- list of lists
    State('miscarriage_dates',  [], list),  # Dates of miscarriages -- list of lists
    State('abortion_dates',     [], list),  # Dates of abortions -- list of lists
    State('short_interval_dates',[], list),  # age of agents at short birth interval -- list of lists

These end up being a pretty long list, I'm wondering whether we might want to refactor these into a dataframe or other format that's easier to work with. Alternatively, if these aren't used often, we could either remove them altogether or place them in an analyzer.