[Rd] ordering dataframe for strata() (PR#13830)

[eric.vanderwal at usask.ca]

I've been using strata(sampling) and found that if the dataframe to be 
sampled ("data") consists of repeated measures that are not sorted in 
order the run will fail on a given stratum.  Ordering the dataframe 
prior to using strata() eliminates this problem.

Thanks,
Eric

-- 
Eric Vander Wal
Ph.D. Candidate
University of Saskatchewan, Department of Biology,
112 Science Place, Saskatoon, SK., S7N 5E2

"Pluralitas non est ponenda sine neccesitate"