DeSTSector: use BrokenLineTrajectory instead of PiecewiseTrajectory

... and make DeSTSensor return LineTraj by value instead std::unique_ptr

Must be applied in conjunction with Lbcom!310 (merged) and Rec!1366 (merged)

Note: this (implicitly) eliminates the (effect of the) bug in PiecewiseTrajectory! (see !1695 (merged))