Arduino BigNumber library port and ATAN

[Chasfield]

Thank you for the range scaling tip. That should speed things up for inverse trig.

I tested my guess refinement ATAN approach for x < 0.1. That too is highly convergent in that range, requiring only a handful of iterations when ATAN (x) = x is used as the first guess.

The best option for the Arduino Uno will be whichever is the most stable. I think that will be the one which stacks the least function calls on top of each other. So I will dust off my Taylor series code and make comparisons.