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.
The function arguments were reviewed to add missing 'const' keywords,
and some classes were modified more deeply to make some pointer class
variables constant.
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.