Started the third chapter of the tutorial on linear solvers.

It is only a first draft and I think it should be reorganized a bit in 2 parts:
1 - a compact table summarizing the main API and its use (this is what would expect an "expert" user)
2 - a discussion about the various algorithm in Eigen to guide the newbies in linear algebra

Currently I mixed the discussion with the API, but it is still better than nothing !