docs: public static DMatrixSparseCSC extractColumn(DMatrixSparseCSC A , int column , @Nullable DMatrixSparseCSC out ) {
  
          if( out == null )
              out = new DMatrixSparseCSC(1,1,1);
  
          int idx0 = A.col_idx[column];
          int idx1 = A.col_idx[column+1];
  
          out.reshape(A.numRows,1,idx1-idx0);
          out.nz_length = idx1-idx0;
  
          out.col_idx[0] = 0;
          out.col_idx[1] = out.nz_length;
  
          System.arraycopy(A.nz_values,idx0,out.nz_values,0,out.nz_length);
          System.arraycopy(A.nz_rows,idx0,out.nz_rows,0,out.nz_length);
  
          return out;
      }