Bayes tracker visualisation improvements and making the mdl tracker optional.

[cdondrup]

• Publishing a pose array for all detected people to have more generic output
• Added missing bayes tracker parameters to launch files and READMEs. Prefixed mdl tracker parameters to be more specific.
• Starting the mdl tracker is now optional when using the robot launch file. with_mdl_tracker:=true starts the mdl tracker in addition to the bayes tracker. Default is false
• The stick figure visualisation is now oriented according to the velocity of the tracker