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
@marqh raised the point about the generic nature of the ValueError and in this case I felt it was appropriate, but I find it a very tricky area. I agree we should append the message from the wrapped exception.