cumulative medication duration

As discussed here: https://groups.google.com/d/topic/pophealth-dev/1Iw48pRe8cw/discussion

A cumulative medication duration is sometimes provided and could be different than the difference between the start_time attributes of medication entries, or the difference between start_time and end_time attributes in a single medication entry. If the duration is provided, it must have been generated by information that exists outside of the scope of start_time and end_time.

Import a provided cumulative medication duration if provided and override a calculated duration.