Adjusted MoveBaseState for input waypoint in PoseStamped instead of deprecated Pose2D.

[tbazina]

Changes explained:

• Replaced a deprecated Pose2D message with PoseStamped for waypoint specification,
• action_topic field is now a state param to allow /move_base in a different namespace (Issue Enhancement: Make action_topic of move_base_state a configurable state param #10),
• Added 'preempted' outcome to handle cases when the navigation is preempted (canceled by another goal).

Linked Issues
Resolves #10, but not the most important change.

[dcconner]

My apologies. This repo fell off my radar.
Given the delay and lack off comments, I am reluctant to merge to master without some follow up.

I am inclined to make these changes to the ROS 2 setup.