Function: isprimepower
Section: number_theoretical
C-Name: isprimepower
Prototype: lGD&
Help: isprimepower(x,{&n}): if x = p^k is a prime power (p prime, k > 0),
 return k, else return 0. If n is present, and the function returns a nonzero
 result, set n to p, the k-th root of x.
Doc: if $x = p^{k}$ is a prime power ($p$ prime, $k > 0$), return $k$, else
 return 0. If a second argument $\&n$ is given and $x$ is indeed
 the $k$-th power of a prime $p$, sets $n$ to $p$.
