Resolve "Add factor topology"

Closes #10 (closed)

include/gnss/factor/factor_gnss_fix_2D.h

@@ -42,6 +42,11 @@ class FactorGnssFix2D : public FactorAutodiff<FactorGnssFix2D, 3, 2, 1, 3, 3, 1,
 
         virtual ~FactorGnssFix2D() = default;
 
+        virtual std::string getTopology() const override
+        {
+            return std::string("ABS");
+        }
+
         template<typename T>
         bool operator ()(const T* const _x,
                          const T* const _o,