High-accuracy approximation of the complex probability function by Fourier expansion of exponential multiplier

The real K(x,y)K(x,y) and imaginary L(x,y)L(x,y) parts of the complex probability function are approximated as rapidly convergent series, based on the Fourier expansion of the exponential multiplier. This approach provides rapid and accurate calculations of the Voigt and complex error functions in the most challenging Humlíček regions 3 and 4.