Add support for alternativeValueSet sub-element of audioObject.

[rsjbailey]

2076-2 adds a new optional sub-element alternativeValueSet on audioObject.

An alternative set of parameters that will be used if the alternativeValueSetID is referenced by an audioProgramme or audioContent element.

The alternativeValueSet sub-element allows an alternative set of parameters for the audioObject to be defined. The parameters defined in this sub-element will take precedence over the same parameters in the parent audioObject element. The parameters defined in the parent audioObject that have not been defined in the alternativeValueSet shall be used in that alternativeValueSet. Multiple alternativeValueSets can be defined in an audioObject to allow multiple variations to be defined.

The parameters of alternateValueSet are

• audioObjectLabel
• audioObjectInteraction
• gain
• headLocked
• positionOffset
• mute

• Is it correct that this is a subelement of audioObject? (i.e. you don't just reference the ID from audioObject)

• How do we avoid too much duplication between this and AudioObject?