Adding UNCLASSIFIED and COINCIDENT classification types.

This is so that geometric intersection can "pre-classify" an
intersection as coincident.

src/bezier/_intersection_helpers.py

@@ -403,6 +403,8 @@ class IntersectionClassification(enum.Enum):
     Provided as the output values for :func:`.classify_intersection`.
     """
 
+    UNCLASSIFIED = -1
+    """The intersection has not yet been classified."""
     FIRST = 0
     """The first curve is on the interior."""
     SECOND = 1
@@ -417,6 +419,8 @@ class IntersectionClassification(enum.Enum):
     """Intersection at a corner, interiors don't intersect."""
     TANGENT_BOTH = 6
     """Tangent intersection, both curves are interior from some perspective."""
+    COINCIDENT = 7
+    """Intersection is actually an endpoint of a coincident segment."""
 
 
 class Intersection(object):  # pylint: disable=too-few-public-methods

src/bezier/surface_intersection.f90

@@ -33,12 +33,14 @@ module surface_intersection
        remove_node, finalize_segment, check_contained
   public &
        Intersection, CurvedPolygonSegment, &
+       IntersectionClassification_UNCLASSIFIED, &
        IntersectionClassification_FIRST, IntersectionClassification_SECOND, &
        IntersectionClassification_OPPOSED, &
        IntersectionClassification_TANGENT_FIRST, &
        IntersectionClassification_TANGENT_SECOND, &
        IntersectionClassification_IGNORED_CORNER, &
-       IntersectionClassification_TANGENT_BOTH, SurfaceContained_NEITHER, &
+       IntersectionClassification_TANGENT_BOTH, &
+       IntersectionClassification_COINCIDENT, SurfaceContained_NEITHER, &
        SurfaceContained_FIRST, SurfaceContained_SECOND, newton_refine, &
        locate_point, classify_intersection, add_st_vals, &
        surfaces_intersection_points, get_next, to_front, add_segment, &
@@ -73,13 +75,15 @@ module surface_intersection
   real(c_double), parameter :: LOCATE_EPS = 0.5_dp**47
   integer(c_int), parameter :: MAX_EDGES = 10
   ! Values of IntersectionClassification enum:
+  integer(c_int), parameter :: IntersectionClassification_UNCLASSIFIED = -1
   integer(c_int), parameter :: IntersectionClassification_FIRST = 0
   integer(c_int), parameter :: IntersectionClassification_SECOND = 1
   integer(c_int), parameter :: IntersectionClassification_OPPOSED = 2
   integer(c_int), parameter :: IntersectionClassification_TANGENT_FIRST = 3
   integer(c_int), parameter :: IntersectionClassification_TANGENT_SECOND = 4
   integer(c_int), parameter :: IntersectionClassification_IGNORED_CORNER = 5
   integer(c_int), parameter :: IntersectionClassification_TANGENT_BOTH = 6
+  integer(c_int), parameter :: IntersectionClassification_COINCIDENT = 7
   ! Values of SurfaceContained enum:
   integer(c_int), parameter :: SurfaceContained_NEITHER = 0
   integer(c_int), parameter :: SurfaceContained_FIRST = 1