addCamera method should be declared as protected

[rabauke]

There is a comment in the source of the class CameraMetaData which reads

// NOTE: *NOT* `final`, could be derived from by downstream.
class CameraMetaData {
public:
  CameraMetaData() = default;

// omitted code

private:
  const Camera* addCamera(std::unique_ptr<Camera> cam);
};

A class that derives from CameraMetaData most likely wants to add camera metadata in its own constructor. For this purpose, grating access to the addCamera method by declaring it as protected would be very convenient. As of the current implementation of CameraMetaData, each derived class needs to implement its own variant of the addCamera method (or implement the functionality of addCamera directly in its constructor).