computes the arc tangent of the argument
Uses the typical taylor series
but may reduce arguments using the following identity
tan(x+y) = (tan(x) + tan(y)) / (1 - tan(x)*tan(y))
since tan(PI/8) = sqrt(2)-1,
atan(x) = atan( (x - sqrt(2) + 1) / (1+x*sqrt(2) - x) + PI/8.0
Set the rounding mode to be one of the following values:
ROUND_UP, ROUND_DOWN, ROUND_HALF_UP, ROUND_HALF_DOWN,
ROUND_HALF_EVEN, ROUND_HALF_ODD, ROUND_CEIL, ROUND_FLOOR.
Breaks a string representation up into two dfp's such
that the sum of them is equivilent to the input string, but
has higher precision than using a single dfp.