Past year model run shouldn't require FUTURE to be specific (e.g. PBA50)

[lmz]

Reported by @alexmitrani, TM1.5 doesn't run smoothly for historic years (e.g. 2015) with FUTURE=NA
but rather requires FUTURE=PBA50 etc, which doesn't really make sense as it's not in the future.

Document what's requiring FUTURE to be set this way and remove the constraint for historic runs.