Use std::ptrdiff_t instead of size_t to iterate an Eigen matrix.
Use std::ptrdiff_t instead of size_t to iterate an Eigen matrix.

billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,sheim/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake