A character vector listing which gmse_apply values should be
reported/summarised. Currently allowable values are
"res","obs","culls","scares","castrations","feeds","helps","tend_crops",
"kill_crops","cull_cost","scare_cost", and "yield". Note that for all
actions and yield, the statistic returned is the sum across users; for
cull_cost and scare_cost the returned values are the mean costs across
users. Can be NULL; if so, if `output` is also
NULL, all possible values are returned. If `output` is not NULL, only the
values already present in `ouput` are returned, in the same column order.