The Anderson Turn behavior is meant to enable a maneuver that it accessible to
the vehicle in virtually any mode. It's conceivable that its run condition is
simply something like DEPLOY=true.

The behavior is engaged with a message to a MOOS variable. While this behavior
supports dynamic parameter updates via a MOOS variable named in the updates
parameter, such updates are meant for modifying default parameters that may be
in play on all turns.

The behavior receives input from another variable, named with the engage_var
parameter. For example:

  engage_var = ATURN_ENGAGE

Postings to this variable may be used to invoke an Anderson turn. For example:

  ATURN_ENGAGE = starboard

or

  ATURN_ENGAGE = starboard, turnrad=20

In the latter example a starboard turn is requested with a turn radius of 20
meters. In the former example, the unspecified turn radius will result in the
default turn radius being used. The default turn radius is set in a separate
behavior configuration parameter "turn_radius=15"

Once invoked the behavior will proceed through a set of waypoints
completing an Anderson turn. When the last waypoint is reached, the
behavior will return to "off" mode. Note that it is still a "running"
behavior from the helm's perspective of having its run conditions
met. It is simplay waiting a new command to turn. Not unlike a running
obstacle avoidance behavior where the obstacle is still outside of
range to take action.

During the turn, the behavior may be given the command:

  ATURN_ENGAGE = off

This will abort the Anderson turn and the vehicle should go back to what it was
doing before the turn, depending on how the mission is configured.