First batch of matrix sum/sum-symmetric methods in linalg framework

- added sum/symmetric sum methods on matrices (currently works with SGMatrix and Eigen::Matrix). Only Eigen3 backend is added in this patch.
- unit tests with both these matrices (with/without diagonal entries) with explicitly set EIGEN3 backend.

Future tasks -
- add viennacl::matrix backend for these methods
- add default global backend unit-tests.