Should we make Borehole a subtype of SF_SamplingFeature ?

[denevers]

This would not change much our instance documents (just forces a mandatory sampledFeature).
I know there is a SF_SamplingCurve too, but it's not strickly a curve here (it's a bit of 19148 machinery). This gives us the hability to have this borehole as a target of a relatedSamplingFeature and link to other sampling features and obs.