Poly-geodesic arc Handling

[efifogel]

Fixed handling poly-curves. Poly-curves are piecewise curves that are not necessarily linear, that is, chains of connected curves of a specific type. Most of fixes in this PR apply to code that handles poly-curves, where the curves are geodesic arcs on the sphere. Observe that polylines (piecewise linear curves) are a specific type of poly-curves.

Summary of Changes

Most of the changes are made in the file Arr_polycurve_basic_traits_2.h of the "2D Arrangements" package.
The class template Arr_polyline_traits_2 derives from Arr_polycurve_traits_2, which in turn derives from Arr_polycurve_basic_traits_2. Each class in this hierarchy adds some functionality.
They all handle poly-curves. The first handles polylines; the other two handle poly-curves. The template parameter SubcurveTraits is referred to as the subcurve traits.

1. Fixed several bugs in the handling of poly-curves, where the curves are geodesic arcs in a sphere. In particular, the SubcurveTraits parameter was substituted by an instance of the template Arr_geodesic_arc_on_sphere_traits_2.
2. Replaced 'typedef' with modern 'using' (in Arr_polycurve_basic_traits_2.h)
3. Improved the inline documentation (in Arr_polycurve_basic_traits_2.h) and applied a general clean up.
4. Fixed the return type of the functors Construct_min_vertex_2 and Construct_max_vertex_2 to be either a point by value or a point by reference depending on the sub-curve traits.
5. Added missing Construct_point_2 functor. (This is not an important addition, because the Construct_point_2 functor of the polycurve traits simply delegates the Construct_point_2 functor of the subcurve traits; nevertheless, it was added for completion.)
6. Fixed small problems in Arr_counting_traits_2 and Arr_tracing_traits. These two template classes are traits decorators that can be used to obtain metadata about the executions of traits functors.

Release Management

• Affected package(s): Arrangement_on_surface_2 (also Surface_sweep_2 in a couple of places, where a specific type was replaced by 'auto' to take advantage of (4.) above.
• Issue(s) solved (if any): NA
• Feature/Small Feature (if any): NA
• Link to compiled documentation (obligatory for small feature): NA
• License and copyright ownership: TAU

[sloriot]

Successfully tested in CGAL-6.0.1-Ic-351