return E(k), the complete Elliptic Function
integral from 0 to pi/2 of 1/sqrt(1-k^2 *(sin(x))^2) dx
uses the arithmetic/geometric mean method. (Abramowitz & Stegun p598)
k must lie in -1 < k <1.
E(k) diverges as log(4/sqrt(1-k^2)) as k->1
interpreted function, defined at i/ellipse.i line 13