Use pd.Timedelta for time_since_treatment

[jbao]

To represent duration in the time_since_treatment column, we have 2 options:

• put numerical values into time_since_treatment and persist the time unit in the metadata [non-intuitive and probably error-prone]
• represent duration directly with the pandas dtype Timedelta [preferred]

[LappleApple]

@jbao Is it OK to label "Help Wanted"?

[jbao]

@LappleApple I think we need more discussion concerning the above 2 options.