lapack/gonum,mat: add Dpotri and use it in Cholesky
Created by: vladimir-ch
Dlauum
has a use after all.
Please take a look.
name old time/op new time/op delta
CholeskyInverseTo/n=10-4 10.8µs ± 2% 3.2µs ± 1% -70.16% (p=0.008 n=5+5)
CholeskyInverseTo/n=100-4 1.07ms ± 2% 0.51ms ± 2% -52.06% (p=0.008 n=5+5)
CholeskyInverseTo/n=1000-4 713ms ± 1% 315ms ± 1% -55.83% (p=0.008 n=5+5)