wrapper for do_* funcs to call process_* with missing handler. Unpacks the FieldKey.

def fkapply(models,pool,fn,missing_fn,(nombre,pkey,field),*args):
  "wrapper for do_* funcs to call process_* with missing handler. Unpacks the FieldKey."
  if (nombre,pkey) in models: return fn(pool,models[nombre,pkey],field,*args)
  else: return missing_fn(pool,field,*args) if missing_fn else ['missing']