mutuable correctness in obj reader and writer

source/RobotAPI/libraries/armem_grasping/client/KnownGraspCandidateReader.h

@@ -58,7 +58,7 @@ namespace armarx::armem::grasping::known_grasps
 
         armem::client::MemoryNameSystem& memoryNameSystem;
         armem::client::Reader memoryReader;
-        std::mutex memoryWriterMutex;
+        mutable std::mutex memoryWriterMutex;
     };
 
 

source/RobotAPI/libraries/armem_objects/client/instance/ObjectWriter.h

@@ -60,7 +60,7 @@ namespace armarx::armem::obj::instance
 
         armem::client::MemoryNameSystem& memoryNameSystem;
         armem::client::Writer memoryWriter;
-        std::mutex memoryWriterMutex;
+        mutable std::mutex memoryWriterMutex;
     };