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
It seems from the logic that we *could* still issue this warning, so why are we now ditching it ?
I thought it was potentially useful (if you believe in warnings at all)