[eigen] Column matrix

[guillaume saupin <guillaume.saupin@xxxxxx>]

Hello,

Is there a kind of sparse matrix in eigen which stores only the non null 
column of a matrix (i.e. a ColumnMatrix) ?

In our libraries, we currently use this kind of matrix and as we plan to 
completely rely on Eigen, we'd like to have this kind of matrix.

As for the SkylineMatrix, we are willing to develop this ColumnMatrix if 
it does not already exists, and if you are ready to integrate it inside 
eigen.

Thanks,

guillaume