Allow user to specify a "turnaround distance" (polygon overflow) on survey missions

[rafaellehmkuhl]

That's specially when you're using a sensor like a sidescan sonar, or a camera, where you need to operate the vehicle a little bit outside of the polygon for the actual polygon to be completely captured.

[vshie]

This is important for missions with a towfish!

[ES-Alexander]

Internal discussion:

Turnaround distance pads the mission polygon in the direction of the survey