Timeseries seconds

@macumber This pull request changes TimeSeries to store integer seconds rather than floating point days. As is, it passes all of the TimeSeries tests but causes failures of a couple of other tests:

* SqlFileFixture.TimeSeriesValues
* FloodPlot.TimeSeriesFloodPlot_Detailed_Year

Since it doesn't pass those tests, I didn't actually modify any of the interpolation functions - those now compute the floating point day deltas and then proceed as before. I think I have either misunderstood something or messed up the arithmetic someplace.