Some new math functions

[mrvux]

Hello, just a small pull request since I use those as extension methods almost everywhere, just thought this would benefit everyone:

• Abs for Vector2 / Vector3
• Some properties for bounding box (size/ center)

Thanks Julien

[bdachev]

Out of curiosity why do you calculate Center that way but not (max+min)*0.5?

[mrvux]

@bdachev Good point indeed, updated it now.

[manofstick]

@mrvux / @bdachev

Not sure if it matters - but the previous way (i.e. min + (max-min)*.5) would have avoided overflow... But maybe that's just an academic point that would never occur in reality... Famous last words?

[xoofx]

Thanks, sorry for the delay (the (min+max)/2 blocked me, but after reading some paper, it is not a huge matter for floats...)