[eigen] Divide & conquer SVD algorithm

[Tristan Sahel <tristan.sahel@xxxxxxxxx>]

Hello,

We are a group of 3 students from the Ensimag, a French computer science 
engineering school. We hope to contribute to the improvement of Eigen as 
part of our end-of-the-year project.

After reading through the todo list, and following our teachers' advice, 
we have come up with a plan.

First, we would need to get used to the code and the specific array 
functionalities, so we thought we would try and develop the "shift" or 
"rotate" functions of the Array Module. We saw that the todo list was 
not up-to-date though, so we would require your confirmation that the 
job is still needed before getting down to it. We are hoping that the 
implementation of these functions will not be too hard or long.

Then, we are planning to continue the work of our fellow students of the 
Ensimag from last year, who were trying to improve the SVD by 
developping a divide & conquer algorithm. Their work was very promising 
and their algorithm was slightly faster than the Jocobi SVD algorithm. 
However, they did not have time to finish the implementation of their 
divide & conquer: they were missing one step, the last one, which they 
explained step by step in their todoBDcsvd.txt document. If we manage to 
finish the job, the calculation time should become o(n^2).

Best regards,
Imène, Bediss, and Tristan.