Raise errors when there are NaNs in the state file

[orianac]

When I feed a state file with missing data in it (in my case it was for daily precipitation from November 1-November 24 for a metsim run of the following January 1 - December 31). Metsim ran without throwing an error, creating a 3-hourly file with missing data from January 1-February 22. Ideally metsim would throw an error instead.