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
no need for the ```, ve``` syntax. The preferred syntax uses ```as```, but in this case you don't use the exception so neither is needed.