BoreholeTrajectory occurence

[mbeaufils]

For now the model define a borehole can have [0, 1] trajectory.

In real life, a borehole always have a trajectory, but because it has not been recorded, 3D geometry may not be available (yet a device could get it ad hoc if the hole still exists).

I suggest to define borehole have 1 trajectory. Trajectory being conceptual, with or without known geometry.