precision issue with rolling_std
i tested `rolling_std` func with some real data (floating number with format x.xx).  For a Series/DataFrame which contains let's say 5 continuous identical rows among several hundreds, the `rolling_std` could return a small number with 10^-7 scale instead exactly zero.  this is probably a known issue which has not been officially documented.  i'm just curious with the reason behind and why the std member func works better.
