Fix BigMath.atan precision safe margin

[tompng]

Increase BigMath.atan(x, prec) precision when |x|>0.5

BigMath.atan(x, prec) is calculated with precision=prec+BigDecimal.double_fig when |x| <= 0.5.
But this safe margin was not properly applied when |x| > 0.5.

BigMath.atan(BigDecimal('0.2'), 100) #=> accuracy was about 116 digits
BigMath.atan(BigDecimal('0.8'), 100) #=> accuracy was about 108 digits
BigMath.atan(BigDecimal('3'), 100) #=> accuracy was about 100 digits