Fix: MathX.Round() - fixing some instances of incorrect rounding and addin…

[tristanjthompson]

During a project a while ago I found an edge case a while ago whereby the MathX.Round() function wasn't matching the output of Excel. At the time I downloaded the NPOI source, made a fix and incorporated it into the project. I've since realised I should have created a PR for it (which is what this is!). I posted the change on Stackoverflow here: https://stackoverflow.com/questions/41467799/npoi-round-formula/41961253#41961253

In summary - the value 481.75478 should round to 481.75, however, it was rounding to 481.76

I have:

• run the "TestRound()" test function and saw it pass
• added the above case to the list of scenarios in "TestRound()" and saw it fail
• added my fix and saw "TestRound()" pass again

[tristanjthompson]

@tonyqus is there any more information you need from me or any changes you'd like me to make in order for this to be merged?

[tonyqus]

If Unit test is passed, I don't have any concern. Thank you for your efforts.