[eigen] alpha4

[Benoît Jacob <jacob@xxxxxxxxxxxxxxx>]

Hi List,

Let me introduce alpha4 to you.

The coolest thing about it is that Gael Guennebaud did most of the work. Since 
i've been working on Eigen (for 18 monthes now), it's the first time that 
someone else is contributing such large improvements.

Here's a summary of what Gael's patches do:

- fix the sizeof issue that was discussed on this list. Done by reintroducing 
a more subtle form of the MatrixStorage class, used as a member in Matrix. By 
the way, fixing a nasty bug that I had introduced when removing 
MatrixStorage.

- add NumTraits for more types.

- fix compilation issues by adding MatrixRef::_ref().

- unify FixedBlock and dynamic-size Block into a single generic Block class.
  Update unit-tests.

- unify + and - into a generic coeff-wise binary op, and add coeff-wise * 
and /.

And here's what I did:

- relicense to dual LGPL3+/GPL2+. So we use only unmodified licenses, and 
allow everybody to use Eigen. More details on updated webpage.

- fix compilation with g++-4.3 (by the way our speed issues with 4.3 are 
almost solved) and prefix the global math functions with ei_ in order to 
avoid risks of conflicts.

- with advice and code snippets from Gael, I reworked (and simplified) Eigen 
to be much better extensible to new scalar types.

There is a lot more to come both from Gael and from me. Great time ahead for 
Eigen!

Cheers,
Benoit

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