Add service to toggle in and out of localization mode. (ros2 branch)

[malban]

Basic Info

Info	Please fill out this column
Ticket(s) this addresses
Primary OS tested on	Ubuntu 18.04
Robotic platform tested on	gazebo simulation of small indoor robot with single plane lidar

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Description of contribution in a few bullet points

See #510

• Added service to base class to enable switching between localization and mapping modes

Example use case would be to explore an area while building up a map in map mode and then switch to localization mode to perform tasks within that area.

Description of documentation updates required from your changes

• Added new service to the README documentation.

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Future work that may be required in bullet points

[malban]

Rebased and updated to publish covariance from #511