Don't use 'using namespace' in C++ header files

[johnwason]

Some header files have using namespace:

https://github.com/rpiRobotics/linear-subproblem-solutions/blob/fd35b3b4abb3cb8baa85b1e62f0011f424fc8d26/cpp/%2Bsubproblem_setups/sp_2E.h#L14

Don't use using namespace in header files. This pollutes any file that includes the header, and can cause unpredictable results. Fully qualify instead, for example use Eigen:;Vector3d.