[eigen] Quaternion and expression template

[Mathieu Gautier <mathieu.gautier@xxxxxx>]

Hi,

I'm currently setting a prototype of our own math library with eigen2 
and more precisely using the Geometry part of Eigen2. In my library, I 
have many rotation composition using quaternion, like :

q = q1.conjugate() * q2 which are implemented through function :

quat_mult_conj(q, q1, q2).

So we avoid any temporary elements due to inversion, multiplication and 
assignation. But I' rather use the first notation if possible, it could 
also remove many of our operations (mult_conj, conj_mult, etc..)

Actually in Eigen2, expression templates are used with MatrixBase 
through the CWise* or productBase classes. Since Quaternion doesn't 
inherited from MatrixBase, it doesn't seem possible to reuse the CWise* 
mecanism. Do I have to provide a new mecanism based to the CWise* or 
productBase ones to use expression template with the Quaternion classes?

If so, these classes will inherit from the new QuaternionBase, are there 
specifics traps to avoid or part of existing code that could be reused ?

--
Mathieu Gautier