allow multiple loudnessMetadata elements in audioProgramme and audioContent

[tomjnixon]

In BS.2076-2, there may be multiple loudnessMetadata elements in audioProgramme and audioContent; see tables 32 and 38,

[tomjnixon]

We're not sure what the API of this should be.

One option would be to wrap this in a class:

struct LoudnessMetadatas {
  using tag = ...;
  std::vector<LoudnessMetadata> data;
};

audioObject.get<LoudnessMetadatas>().get();
audioObject.set(LoudnessMetadatas{std::vector<LoudnessMetadata>{...}});

... or to just use a std::vector<LoudnessMetadata> directly:

using LoudnessMetadatas = std::vector<LoudnessMetadata>;
audioObject.get<LoudnessMetadatas>();
audioObject.set(std::vector<LoudnessMetadata>{...});

... or to add an API to AudioObject to add and remove LoudnessMetadatas:

std::shared_ptr<LoudnessMetadata> loudnessMetadata = LoudnessMetadata::create();
audioObject.add(loudnessMetadata);
audioObject.remove(loudnessMetadata);
audioObject.getLoudnessMetadata() -> std::vector<std::shared_ptr<LoudnessMetadata>>

I think we both prefer option 2.

[tomjnixon]

The decision here should probably apply to #90, too

[rsjbailey]

Elsewhere in the library where there is this sort of relationship (for example
SpeakerLabels in AudioBlockFormatDirectSpeakers) the plural type is just an alias for std::vector, then there are add() and remove() methods for the singular type in the parent.

[tomjnixon]

Oh yeah! I tried to think of other cases like that, but didn't spot that one.

Thanks,