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
I'd like to see a `self.assertRaisesRegexp(ValueError, 'not the expected coord name'):` or similar. Same above but with `'multiple coord names'` or similar. This lets us check the code is failing as expected rather than somewhere else.