=======================================
The SparseQR is set as default method. However, in accordance with the previous commit, only if a new enough Eigen library is present will SparseQR be executed.
If the library is old and SparseQR can not be reliably used, FC will automatically fall back to use DenseQR.
This ensures that users that have a new enough library > 3.2.2 will benefit from the performance increase, while the users that use older library will still have
the same performance as in legacy FC.
4b03c3d41b