Specify rounding mode as half to even

[kgryte]

This PR

• specifies the rounding mode as half to even, rather than half away from zero. This follows NumPy and Python 3.x, rather than C and Python 2.7 round half away from zero behavior.

[rgommers]

Thanks for the fix @kgryte.

includes a TODO to consider whether round should support alternative rounding modes

My preference would be not to bother, since this is mostly a niche detail and current libraries don't implement multiple options.

I did a quick scan of NumPy issues, and while this question (e.g., why does round(0.5) give 0?) is asked multiple times, no one argues an alternative matters to them.

For reference, here is the rationale for the rule: numpy/numpy#3382 (comment), Round-0.5-to-the-nearest-even, the rule numpy uses, is usually considered the best option for serious numerical code because it rounds up just as often as it rounds down, thus avoiding a source of statistical bias.

[kgryte]

Removed TODO.

[rgommers]

Thanks, merged!