#5693 elliptic_pi does not compute the principal value

[Raymond Toy]

elliptic_pi does not produce the principal value. See the mailing list for further discussions, but

elliptic_pi(1.5, float(%pi/2), 0.5);
-0.4567203134529103

Wolfram says it should be -0.45672031345 - 2.72069904635*i.

So we have the correct real part but don't compute the imaginary part.