Better support for handling the "last day"

[bknueven]

@dguittet points out that if you start the RTS-GMLC simulation on 12/31/20 and use a 4-hour look-ahead SCED that Prescient will crash on the SCED at 21:00.

This is because there is no data to read for 01/01/2021 at 00:00.