sub method
Subtract a from this matrix
Implementation
Mat33 sub(Mat33 a, [Mat33? b ]) {
if( b != null ) return subMatrixs( a, b );
List<double> e = elements, te = a.elements;
e[0] -= te[0]; e[1] -= te[1]; e[2] -= te[2];
e[3] -= te[3]; e[4] -= te[4]; e[5] -= te[5];
e[6] -= te[6]; e[7] -= te[7]; e[8] -= te[8];
return this;
}