[eigen] Skyline matrix

[guillaume saupin <guillaume.saupin@xxxxxx>]

Hello,

We are planning to use your library in our projects, but we need a 
skyline matrix. Therefore I'd like to implement one for eigen, but I 
don't now where to start.

Is there a specific class that can be a good starting point / skeleton 
to start with ? The SparseMatrixBase might be a good choice.
Should this SkylineMatrix inherit from SparseMatrixBase, or be a 
separate class ?

Thanks,

guillaume