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
> Line 435 is now redundant. Remove it, squash and I'll merge.



> Line 435 is now redundant. Remove it, squash and I'll merge.

done