TrajectoryReferent Union should only concern feature and position

[sgrellet]

In both cases there is a need for location: TrajectoryPosition

[sgrellet]

Leuvent meeting note : A borehole collection has a referent made of either feature or geometry to locate the origin and a location to a TrajectoryLocation pointing to a curve and the distance along linear reference system.
The union prevented to express this. This union was intiallly added by us (not in 19148). We shall delete it

[sgrellet]

done in BhML Model - rev 14

[sgrellet]

after webconf 24 : OCL constraint not to have feature & position NULL at the same time

[sgrellet]

done in BhML Model - rev 18

[sgrellet]

there is a caveat in the model : the geometry information provided could be different betweeb the 'feature' and the 'position' attributes.