Missing complex number function Arg(X)

[coatless]

Complex sugar ops have support for the following R equivalents:

• Re
• Im
• Mod
• Conj

but no support for the Arg function that determines the angle between the real and complex plane (counter-clockwise).

Given the availability of the cpp port of the fortran atan2 inverse function, the base R imp of Arg is easy to mirror. Two-liner addition to the complex header:

inline double complex__Arg( Rcomplex x ){ return ::atan2(x.i, x.r); }
RCPP_SUGAR_COMPLEX( Arg, double )