X_coordinate -> Coordinate

Curved_kernel_via_analysis_2/Point_2.h:   // TODO move to SfP_2l (rebind)
Curved_kernel_via_analysis_2/Point_2.h    // TODO concept! aymptotic

Curved_kernel_via_analysis_2/Make_x_monotone_2.h:   // TODO add operator for non-x-monotone arc

Curved_kernel_via_analysis_2l/Surface_arc_2l.h: // TODO add sanity checks for sheet_at_min/max wrt sheet (eriC)
Curved_kernel_via_analysis_2l/Surface_arc_2l.h:  // TODO implement _check_surface_arc_interior

Quadrical_kernel_via_analysis_2.h: // TODO what to do with Is_bounded_2 and Is_on_2? (eriC)
Quadrical_kernel_via_analysis_2.h: // TODO special handling for vertical arc on upper sheet

Curved_kernel_via_analysis_2l/Curved_kernel_via_analysis_2l_functors.h:        // TODO implement Is_on_2 with Curve_2 == Surface_3 (eriC)