Implement get_vertex(handle)

This is a shorthand to check if a `FixedVertexHandle` points to a valid vertex.

src/delaunay_core/dcel.rs

@@ -133,6 +133,10 @@ impl<V, DE, UE, F> Dcel<V, DE, UE, F> {
         self.faces.truncate(1); // Keep outer face
     }
 
+    pub fn get_vertex(&self, handle: FixedVertexHandle) -> Option<VertexHandle<V, DE, UE, F>> {
+        (handle.index() < self.vertices.len()).then(|| self.vertex(handle))
+    }
+
     pub fn map_vertices<M, V2>(self, f: M) -> Dcel<V2, DE, UE, F>
     where
         M: Fn(V) -> V2,

src/triangulation.rs

@@ -322,6 +322,19 @@ pub trait Triangulation: Default {
         self.s().fixed_vertices()
     }
 
+    /// Get a vertex by its index
+    ///
+    /// To get the handle, wrap your local index in a FixedVertexHandle:
+    /// `let handle = FixedVertexHandle::from_index(my_index);`
+    #[allow(clippy::type_complexity)]
+    fn get_vertex(
+        &self,
+        handle: FixedVertexHandle,
+    ) -> Option<VertexHandle<Self::Vertex, Self::DirectedEdge, Self::UndirectedEdge, Self::Face>>
+    {
+        self.s().get_vertex(handle)
+    }
+
     /// An iterator visiting all faces.
     ///
     /// The first returned face is the outer face, all other faces will be inner faces.