[eigen] BDCSVD update

[Jitse Niesen <jitse@xxxxxxxxxxxxxxxxx>]

Hello,

I continued work on the BDCSVD (bidiagonalizing divide-and-conquer SVD), 
building on the excellent work of the group of students at ensimag in 
Grenoble. The basic algorithm is now fully implemented. It took me longer 
than expected; small mistakes and short-cuts that I took lead to loss of 
accuracy. However, at the moment all tests pass, it seems accurate on 
random matrices, and it's faster than JacobiSVD (about a factor of four 
for a 160-by-160 matrix).

I used the bisection method to solve the nonlinear equation at the heart 
of the algorithm, which is hardly state-of-the-art. In fact, I think most 
of the lines of code I added can be improved, so I intend to spend some 
time optimizing and fine-tuning the BDCSVD class.

Thanks again to Gauthier, Ncolas, Jean and Pierre for their work. Their 
well-documented code and their report made it easy for me to get started.


Jitse