Rename TrajectoryReferent

[sgrellet]

From the discussion in #68 it seems the term 'Trajectory' in the name itself is a bit misleading.
Could be renamed into 'LRSReferent' for example

[rhaener]

LRSReferent actually is something like "green grass": a Referent of a Reference System. If the term "Trajectory" is misleading, why not for example use the term "BoreholeReferent" or even simpler "Referent", because the context Borehole is clear from the enclosing element "BoreholeLRS". However, that what is referenced, the casing for example or drill runs, should be uniquely identified. A Borehole Trajectory or Path could be a valid identification