Make all `setRotationMatrix()` takes constant reference

[AAArthurLiu]

The functions setRotationMatrix() of Sim3 and Sim2 takes a reference instead of a constant reference (currently in Sim3):

void Sophus::Sim3Base< Derived >::setRotationMatrix(Matrix3<Scalar>& R);

Is there a particular reason? Because its implementation calls rxso3().setRotationMatrix(R);, which just needs a constant reference. Using a const reference would be more consistent and convenient.