Fix use of smart pointers in `Trk::RectangularSegmentation'

The previous commit to this class incorrectly used smart pointers,
specifically, their ownership was not released. Moreover, the wrapped
objects were initialised incorrectly, leading to the same
`Trk::PlaneSurface' being duplicated, filling the vector with the same
surface.

Although it appears the owning objects free memory, we play it safe by
keeping the smart pointers here, as opposed to resorting back to raw
ones.


Former-commit-id: d79f6040