Check for variables with no variation

[privefl]

Do it in scaling functions?

This is a problem when using e.g. big_randomSVD() because it would either stop with error TridiagEigen: eigen decomposition failed or run for an infinite time.

Reprex:

library(bigstatsr)
X <- FBM(20, 20, init = rnorm(400))
svd <- big_randomSVD(X, big_scale())
X[, 1] <- 0
svd2 <- big_randomSVD(X, big_scale())