Function demixed_pca 

pub fn demixed_pca(
    conditions: &[Vec<&[i32]>],
    n_components: usize,
    bin_size: usize,
) -> (Vec<f64>, Vec<f64>)

Demixed PCA. Kobak et al. 2016.

conditions: list of (condition trains) where each is a Vec<&[i32]>. Returns (projected, explained_variance_ratio).