mathext: add elliptic integrals (Carlson and Legendre).
Created by: reconditematter
Added Carlson's symmetric elliptic integrals RF and RD. Added Legendre's elliptic integrals of the 1st and 2nd kinds.
Created by: reconditematter
Added Carlson's symmetric elliptic integrals RF and RD. Added Legendre's elliptic integrals of the 1st and 2nd kinds.