inversion of X'X
[xpxi]=invxpx(x)
* x = a (nxk) real matrix
* xpxi = the (k x k) matrix equal to inv(x'x)
x=grand(30,3,'nor',0,1) y=x*[0.2;0.4;0.35]+grand(30,1,'nor',0,1) m = invxpx(x)