Function: qfperfection
Section: linear_algebra
C-Name: qfperfection
Prototype: G
Help: qfperfection(G): rank of matrix of xx~ for x minimal vectors of a Gram
 matrix G.
Doc: $G$ being a square and symmetric matrix with integer entries
 representing a positive definite quadratic form, outputs the perfection rank
 of the form. That is, gives the rank of the family of the $s$ symmetric
 matrices $v{^{t}}v$, where $v$ runs through the minimal vectors.

 A form is perfect if and only if its perfection rank is $d(d+1)/2$ where
 $d$ is the dimension of $G$.

 The algorithm computes the minimal vectors and its runtime is exponential
 in $d$.
