faster matrix multiplication for leaf prediction

[bcjaeger]

Taking sub-matrix views will likely be less fast than iterating over the relevant columns/rows.

Consider implementing this function for data class:

arma::vec submat_mult_lincomb(arma::mat& x,
                              arma::uvec& x_rows,
                              arma::uvec& x_cols,
                              arma::vec& beta){

 arma::vec out (x_rows.size());
 arma::uword i = 0;
 arma::uword j = 0;

 for(auto row : x_rows){
  j=0;
  for(auto col : x_cols){
   out[i] += x.at(row, col) * beta[j];
   j++;
  }
  i++;
 }

 return(out);

}

[bcjaeger]

Fixed with #20