grib save statistic

Alterations to saving logic for time processed statistics

Now:
  if time processed and stat known
    set value
  elif time processed and stat not known
    set value to 'missing
  elif not really a time processed cell method (else)
    raise exception
> (if you believe in warnings at all)

I don't.  At least not in most cases, i don't think this is a case where a warning will help.  However if it's viewed as important I will defer