Process WEPP's soil moisture data

[akrherz]

Over the years, I've often dreamed about a formal processing and databasing of WEPP's soil moisture data. We've had a couple of small proposals to get funding to do just that, but were not funded. I also boggled how such information could be summarized given a large variability within a single HUC12, see this video.

Ultimately, storing the simple mean is hopefully better than nothing and I have some immediate needs #67 to have this available. So might as well get the ball rolling on this task.