SE3vel convention update 

the function SE3vel::Exp(const Mat91& xi) needes to be updated 
`Mat31 phi = xi.head(3);
    Mat31 v = xi.segment<3>(3);
    Mat31 t = xi.tail(3); ` 
     t, v should be exchanged here to maintain the convinetion 
` phi,
 t,
 v ' 
