Update README to include JSON library

README.md

@@ -100,9 +100,9 @@ following packages.
 
 -   spdlog <https://github.com/gabime/spdlog>
 -   suitesparse <http://faculty.cse.tamu.edu/davis/suitesparse.html>
--   cereal <https://uscilab.github.io/cereal/index.html>
 -   Qt5 <http://qt-project.org>
 -   libQGLViewer <http://www.libqglviewer.com>
+-   JSON for Modern C++ <https://github.com/nlohmann/json>
 
 On Ubuntu / Debian these dependencies are resolved by installing the
 following packages.

g2o/core/CMakeLists.txt

@@ -57,11 +57,8 @@ set_target_properties(core PROPERTIES
   SOVERSION ${G2O_LIB_SOVERSION})
 target_link_libraries(core PUBLIC stuff Eigen3::Eigen)
 target_link_libraries(core PUBLIC g2o_ceres_ad)
-if (TARGET cereal::cereal)
-target_link_libraries(core PUBLIC cereal::cereal)
-endif()
 if (TARGET nlohmann_json::nlohmann_json)
-target_link_libraries(core PRIVATE nlohmann_json::nlohmann_json)
+  target_link_libraries(core PRIVATE nlohmann_json::nlohmann_json)
 endif()
 target_compile_features(core PUBLIC cxx_std_17)
 

g2o/core/io/io_wrapper_json.h

@@ -47,7 +47,7 @@ NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(AbstractGraph::AbstractEdge, tag, ids,
 namespace json {
 
 /**
- * @brief Versions of the IO API for the graph using cereal.
+ * @brief Versions of the IO API for the graph. Currently not stored.
  */
 enum class IoVersions {
   kGraph = 0,