add const to set comparison operators to fix #208

DESCRIPTION

···
 Package: dodgr
 Title: Distances on Directed Graphs
-Version: 0.2.20.035
+Version: 0.2.20.036
 Authors@R: c(
     person("Mark", "Padgham", , "[e-mail]", role = c("aut", "cre")),
     person("Andreas", "Petutschnig", role = "aut"),

codemeta.json

···
   "codeRepository": "https://github.com/ATFutures/dodgr",
   "issueTracker": "https://github.com/ATFutures/dodgr/issues",
   "license": "https://spdx.org/licenses/GPL-3.0",
-  "version": "0.2.20.035",
+  "version": "0.2.20.036",
   "programmingLanguage": {
     "@type": "ComputerLanguage",
     "name": "R",

src/pathfinders.h

···
 
 struct by_wt
 {
-    bool operator () (const DijkstraEdge& lhs, const DijkstraEdge& rhs)
+    bool operator () (const DijkstraEdge& lhs, const DijkstraEdge& rhs) const
     {
         if (fabs (lhs._wt - rhs._wt) < 1.0e-12)
             return lhs._i < rhs._i;

src/turn_penalty.h

···
 // https://stackoverflow.com/questions/6989100/sort-points-in-clockwise-order
 struct clockwise_sort
 {
-    bool operator () (const OneEdge &a, const OneEdge &b)
+    bool operator () (const OneEdge &a, const OneEdge &b) const
     {
         if (a.x >= 0.0 && b.x < 0.0)
             return true;