cpp/numeric/special functions/riemann zeta

@1-3@ Computes the of. @A@ Additional overloads are provided for all integer types, which are treated as.

Return value
If no errors occur, value of the Riemann zeta function of, $ζ(num)$, defined for the entire real axis:
 * For $num>1$, $Σ∞ n=1n-num$
 * For $0≤num≤1$, $Σ∞ n=1 (-1)n n-num$
 * For $num<0$, $2num πnum-1 sinΓ(1−num)ζ(1−num)$

Error handling
Errors may be reported as specified in math_errhandling.
 * If the argument is NaN, NaN is returned and domain error is not reported