addEqual method
Add m to this matrix
Implementation
Mat33 addEqual(Mat33 m){
List<double> te = elements, tem = m.elements;
te[0] += tem[0]; te[1] += tem[1]; te[2] += tem[2];
te[3] += tem[3]; te[4] += tem[4]; te[5] += tem[5];
te[6] += tem[6]; te[7] += tem[7]; te[8] += tem[8];
return this;
}