divergence value is not optional

[tomjnixon]

it's currently declared as boost::optional<Divergence> and which is checked by isDefault, but the divergence value is stored in the element contents like this:

<objectDivergence azimuthRange="60.000000" positionRange="0.250000">0.500000</objectDivergence>

so it's impossible to leave it out except by omitting the whole element, but that would be represented by a defaulted ObjectDivergence in AudioBlockFormatObjects.