()
added additional options to Timer.compare and Timer.compare_many. It's easier to
compare different attributes (e.g., total time instead of average time).
arsenal.math.compare should no longer give NaNs for "mean relative error" and
"mean rescaled error".
tweaks to arsenal.math.compare

 - added todo items

 - improved printing of finiteness-check errors

 - added scatter plot option (shows regression line and target line)