Make ReferencePoint's Point3D copy constructor explicit. This implied
the following changes:
* Stock::get_reference_point(ReferencePoint&) becomes
get_reference_point(Point3D&). Reference points were always
retrieved by their coordinates only in the Stock anyway.
* Stock::find_create_reference_point(Point3D&) was added, since it can
have a different outcome than its (ReferencePoint&) counterpart.
* The unit tests for Stock were improved.
To be consistent with the other modules, the positioning server must be
called OwlPS Positioner instead of Positioning, and the executable
owlps-positionerd (even if the daemon mode is not implemented yet).
That's a big commit for a tiny change.