ENH: Add warning for bad rt_chisq

Ready for discussion/review/merge. I just used 3*SD. @drammock maybe you can try on some of your datasets to see if it works okay? I can't remember what use case drove us to look at this exactly... I *think* it might have been one of mine where I had an absurdly large (and incorrect) RT that broke the calculation, which is reflected in the test.

Closes #134.