Add a default writing user_id to the config

core-writer/include/config.hpp

@@ -19,6 +19,8 @@ namespace config
     extern std::string raw_image_dataset_name;
 
     extern uint32_t parameters_read_retry_interval;
+
+    extern int default_writing_user_id;
 }
 
 #endif

core-writer/src/config.cpp

@@ -22,4 +22,7 @@ namespace config {
     hsize_t dataset_increase_step = 1000;
     // To which value to initialize a dataset size.
     hsize_t initial_dataset_size = 1000;
+
+    // -1 means to stay with the same user_id.
+    int default_writing_user_id = -1;
 }