docs: public static void subtractEquals(DMatrixD1 a, DMatrixD1 b)
      {
          if( a.numCols != b.numCols || a.numRows != b.numRows ) {
              throw new MatrixDimensionException("The 'a' and 'b' matrices do not have compatible dimensions");
          }
  
          final int length = a.getNumElements();
  
          for( int i = 0; i < length; i++ ) {
              a.data[i] -= b.data[i];
          }
      }