Known Eigen bugs

[bchretien]

This issue list known Eigen bugs that may impact RobOptim users:

• [ ] Crash when accessing sparse matrix row - fixed in Eigen's 3fcd51f0b38a (v3.2)
• [ ] Error with sparse vector and transpose of matrix - fixed in Eigen's e99ca53c931b
• [ ] MatrixXd block, VectorXd and noalias - fixed in Eigen's 8b0fa51cef5e (v3.2)

cc @jorisv