No pose covariances in factor graph + map acquisition service (mapping module)

[nashj]

This is because nav_msgs/Path, used to communicate path changes from the optimisation to mapping modules, does not contain covariances. A quick fix would be a new custom message for communicating the path changes (note would not be automatically visualisable in rviz), however there is a larger code structure redesign I am working on, which would make the graph manager readble by the mapping class, and communicating path changes would no longer be necessary. If this feature becomes important, the quick fix can be implemented. Otherwise this will wait for the code redesign.