Make `X_original` constant

[Banana1530]

Any reason these are non-const?

Originally posted by @michaelweylandt in https://github.com/_render_node/MDE3OlB1bGxSZXF1ZXN0UmV2aWV3Mjc4MDgzMjQ0/pull_request_reviews/more_threads

[Banana1530]

The follow code snippet should explain the semantics of these seemingly confusing variables.

class MoMA
{
  private:
    // ......
    arma::mat X;          // on X we perform the algorithm

    arma::mat X_working;  // keep track of deflated matrices
    arma::mat Y_working;

    arma::mat X_original;  // const
    arma::mat Y_original;
}