Resolve "Work on const / non-const in wolf base classes"

Closes #184 (closed)

include/core/landmark/landmark_base.h

@@ -90,7 +90,7 @@ class LandmarkBase : public NodeBase, public std::enable_shared_from_this<Landma
         unsigned int getHits() const;
         const FactorBasePtrList& getConstrainedByList() const;
 
-        MapBasePtr getMap();
+        MapBasePtr getMap() const;
         void link(MapBasePtr);
         template<typename classType, typename... T>
         static std::shared_ptr<LandmarkBase> emplace(MapBasePtr _map_ptr, T&&... all);
@@ -124,7 +124,7 @@ std::shared_ptr<LandmarkBase> LandmarkBase::emplace(MapBasePtr _map_ptr, T&&...
     return lmk;
 }
 
-inline MapBasePtr LandmarkBase::getMap()
+inline MapBasePtr LandmarkBase::getMap() const
 {
     return map_ptr_.lock();
 }