Arduino BigNumber library port and ATAN

[Chasfield]

I applied Paul's range reduction and scaling tips to my Taylor series and guess refinement versions of the ATAN function. Taylor wins - ATAN response time is almost instantaneous using 12 terms in the series, though the guess refinement approach was much improved, with only 7 iterations required for good accuracy.

Attached is an RPN-duino screen grab showing the calculator forensic test result in the Y reg and ATAN (1) in the X.

Cheers, Paul