docs: public static double trace( DMatrixSparseCSC A ) {
          double output = 0;
  
          int o = Math.min(A.numCols,A.numRows);
          for (int col = 0; col < o; col++) {
              int idx0 = A.col_idx[col];
              int idx1 = A.col_idx[col+1];
  
              for (int i = idx0; i < idx1; i++) {
                  if( A.nz_rows[i] == col ) {
                      output += A.nz_values[i];
                      break;
                  }
              }
          }
  
          return output;
      }