Trunk 4446: Support deriving auto_expire_date from duration and duration units for Drug Orders

Changes for ticket https://issues.openmrs.org/browse/TRUNK-4446
If I prescribe something with a duration of "N times" that means you need to calculate the duration as something like N * 1/frequency. I haven't looked closely at this line of code, but it's quite possibly doing the right thing.

E.g. if the frequency = twice per day, and duration = 5 times, then the auto expire will be:

    start + 5 * (1 / (2/day))
    start + 2.5 days

Since our frequency units define their coded frequency in terms of days, then it makes sense to me that the code is written in terms of days.

Verify the tests and trust their output. :-)