Eigen loadMarket() and Eigen::saveMarket() doesnot support symmetric matrix

Laplacian is a ubiquitous tool in physics. It can be mimicked discretely in the form of a symmetric matrix. This type of matrix implementation saves a lot of memory space if it's a very very large matrix. It would be better if loadMarket() and saveMarket() API supports symmetric property. I would strongly recommend these two API needs to be in the main codebase of Eigen library rather than considered unsupported APIs.

Note: Python scipy lib supports it too.