Voigt funtcion approximation - Humlicek Region 1

This code approximates the Voigt function in the difficult Humlicek Regions 3 and 4.

This is an approximation of the Voigt function within the Humlicek regions 3 and 4. The approximation is one given by S.M. Abrarov et. al. "High-accurace approximation of the complex probability function by
Fourier expansion of exponential multiplier" (2010).
x = sqrt(ln(2))*(nu - nu0)/alphaD
y = sqrt(ln(2))*alphaL/alphaD
where 'ln' denotes the natural log, nu the wavenumber, nu0 the wavenumber at center, alphaD and alphaL the Doppler and Lorentzian half-width at half-maximum. Suggested values for N & tau are 23 and 12 respectively.

Note: This approximates the Voigt function, not the Voigt Profile.