remove date-checking for state data

[andywood]

users must ensure the timestamps on their state data match the start of the input data, but this might not be desired in some applications -- e.g., 1) one doesn't have data before the start of a forcing dataset, requiring the user to grab data from elsewhere and then modify the time stamp. 2) in an ESP setting, one might prepend one particular year repeatedly to other years.

the fix would be to read but ignore the time field in the state file, and just back-extend the time from the forcings to overwrite it if needed.