[eigen] ComplexEigenSolver with real matrices

[Jitse Niesen <jitse@xxxxxxxxxxxxxxxxx>]

Hello,

I am looking in more detail to the Eigenvalues module, since I'm using it 
in the MatrixFunctions module.

One thing I noticed is that ComplexEigenSolver<Matrix3f> does not work, 
because Matrix3f is a real matrix. Is this supposed to work? The many 
typedefs, distinguishing between real and complex scalars, seem to 
indicate so, and it shouldn't be hard to make it work. But if it's not 
supposed to work, I can add a note in the documentation.

The other thing is that the code is, ummm, not as tidy as the rest of the 
Eigen code base. I also noted the item in the todo list for 3.0: "The 
implementation borrowed from JAMA should be rewritten in a clean way 
separating the Real Schur decomposition as for the complex case. Then the 
ComplexEigenSolver and EigenSolver should be merged." Any objections if I 
start refactoring the code, add documentation, etc? As you know, I work 
slowly so I'm unlikely to make the 3.0 deadline.

Cheers,
Jitse