[eigen] backporting the vectorized quaternion product to 2.0 ?

[Benoit Jacob <jacob.benoit.1@xxxxxxxxx>]

Hi,

We have users (at Celestia) who are very interested in the performance
gain from Rohit's vectorized quaternion product, and are considering
backporting it themselves.

I'm OK personnally to backporting it to 2.0, I can do it, i'd just
like to know if anybody has a strong feeling against doing that, or if
there's a big technical difficulty there.

Rationale:
1) it's not dangerous as long as it's covered by a unit test
2) it's a cool feature and as the next major version takes a long time
to prepare, it's nice to backport some cool features.

Benoit