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
This is me being fussy, but I'd change `ve` to just `e`. If we do move to a different exception type `ve` no longer makes sense.