Special Functions: add Mittag-Leffler function

[diluculo]

The Mittag-Leffler functions were added:

• Mittag-Leffler function: E(α)(z) = sum(k=0)^∞ z^k/Γ(k α + 1)
• generalized Mittag-Leffler function: E(α, β)(z) = sum(k=0)^∞ z^k/Γ(k α + β)
• three-parameter Mittag-Leffler function: E(α, β, γ)(z) = sum(k=0)^∞ (z^k Γ(k + γ))/(k! Γ(γ) Γ(k α + β))

I translated Professor Robert Garrappa's Matlab code (BSD-3-Clause).

[cdrnet]

Thank you very much! And sorry for the late response.