File writer and spot finding improvements

frame_serialize/CBORStream2Serializer.cpp

@@ -299,7 +299,7 @@ inline void CBOR_ENC_START_USER_DATA(CborEncoder& encoder, const char* key,
                                      const StartMessage& message) {
     nlohmann::json j;
     j["file_prefix"] = message.file_prefix;
-    j["data_file_count"] = message.data_file_count;
+    j["images_per_file"] = message.images_per_file;
     j["source_name"] = message.source_name;
     j["source_name_short"] = message.source_name_short;
     j["instrument_name"] = message.instrument_name;