docs: public static DMatrixRMaj sumCols(DMatrixSparseCSC input , @Nullable DMatrixRMaj output ) {
          if( output == null ) {
              output = new DMatrixRMaj(1,input.numCols);
          } else {
              output.reshape(1,input.numCols);
          }
  
          for (int col = 0; col < input.numCols; col++) {
              int idx0 = input.col_idx[col];
              int idx1 = input.col_idx[col + 1];
  
              double sum = 0;
              for (int i = idx0; i < idx1; i++) {
                  sum += input.nz_values[i];
              }
              output.data[col] = sum;
  
          }
  
          return output;
      }