Update interval schedule forward translation to use seconds

@macumber This pull request finishes up the schedule/time series work we started a while back. I updated both the fixed interval schedule and the variable interval schedule to use seconds rather than days in forward translation. One thing that came out in the process is that the variable interval schedule was using floating point days internally and losing a couple of minutes every day in an 8760 schedule. I changed that to use seconds, but I was wondering if it shouldn't just use DateTime objects directly.

The "interpolate" field is still not written out, but I haven't been able to figure out from the documentation what the correct behavior is.