[eigen] small update

[Benoît Jacob <jacob@xxxxxxxxxxxxxxx>]

Hi List,

- I spotted an error in the CMakeLists that was caused by search-and-replace, 
and that prevented Eigen2 from installing correctly. That's fixed now.

- Also, I improved the unit-tests a bit today. There are now comments.

- I also remember that someone (Konstantin?) asked why the Matrix(int,int) 
constructor had default argument values set to 1. That's changed now. There 
are no longer default values. Instead, there are now Matrix() and Matrix(int) 
constructors, and we no longer default to 1 for dynamic dimensions. All 
dynamic dimensions are now required to be specified at construction-time.  
Having separate constructors instead of just default values allows us to 
produce compile errors, instead of failed asserts at runtime, which is better 
here. This means that

MatrixXd m; // defaults to 1 row, 1 col

no longer compiles. If an initial size of 1,1 is really what you want, do:

MatrixXd m(1,1);

- someone proposed on IRC that Object should be renamed to MatrixBase. What do 
you think? 

Cheers,
Benoit

P.S. I've not been fully idling, as I've been thinking a lot and I think 
whenever I come back full-force to coding (mid-dec.), we'll quickly reach the 
point where porting apps from Eigen 1 to 2 is possible.

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