Function: ispower
Section: number_theoretical
C-Name: ispower
Prototype: lGDGD&
Help: ispower(x,{k},{&n}): true (1) if x is a k-th power, false (0) if not.
 If n is given and a k-th root was computed in the process, put that in n.
 If k is omitted, return the maximal k >= 2 such that x = n^k is a
 perfect power, or 0 if no such k exist.
Description: 
 (int):small       isanypower($1, NULL)
 (int, &int):small isanypower($1, &$2)

