Move t=0 output (for INSTANT output) to its own file

Talking offline with @rljacob , I found out that the way we save INSTANT output is not the preferred way. We currently save t=0 in the first set of files.

Solution: if the output file has storage_type=NumSnaps, save t=0 in a file with capacity 1snap, and close it. If the storage type is "Monthly" or "Yearly", I would still keep t=0 in the same file as other snaps.