docs: public static double diffNormF(DMatrixD1 a , DMatrixD1 b )
      {
          if( a.numRows != b.numRows || a.numCols != b.numCols ) {
              throw new IllegalArgumentException("Both matrices must have the same shape.");
          }
  
          final int size = a.getNumElements();
  
          DMatrixRMaj diff = new DMatrixRMaj(size,1);
  
          for( int i = 0; i < size; i++ ) {
              diff.set(i , b.get(i) - a.get(i));
          }
          return NormOps_DDRM.normF(diff);
      }