Fix/scaling

[GFabien]

When scale = FALSE and scale_block = TRUE, each block x is normalized by ||x||_F / sqrt(N) where N is either the number of rows of x or the number of rows minus one. This PR modifies the code to apply this transform in a more straightforward fashion. Here is a small benchmark to show that it does not only simplify the code but also speeds up the computation:

library(rbenchmark)

custom_frob = function(x, N, bias = T) {
    if(NROW(x) > NCOL(x))
    {
        covarMat <- cov2(x, bias = bias)
    }
    else
    {
        covarMat <- 1/N*(x%*%t(x))
    }
    variance_block <- sum(diag(covarMat))
}

x = matrix(rnorm(10000), 500, 1000); bias = T; N = nrow(x) + bias - 1 
benchmark("new" = {
    res = 1/sqrt(N) * norm(x, type = "F")
},
"old" = {
    res = sqrt(custom_frob(x, N, bias))
},
replications = 1000)

>   test replications elapsed relative user.self sys.self user.child sys.child
> 1  new         1000   1.193    1.000     1.194    0.000      0.000     0.000
> 2  old         1000 136.393  114.328   135.085    1.129      0.022     0.173