Support more than unit durations
Support more than unit durations

Previously we could only specify durations in unit lenths,
e.g. 1 week, 1 month, 1 year. This allows the specification
of integral lengths, e.g. 2 weeks, 3 months, etc.

WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed,WikiWatershed/model-my-watershed