Make sure spec handling of zero-order ambisonics matches reference implementation

[sunghee-hwang]

We have channel-based Mono. Do we need zero-order Ambisonics too?

[tdaede]

We can update the spec to match what the current reference software does with zero-order Ambisonics (treat it as stereo).

[cto300]

We can update the spec to match what the current reference software does with zero-order Ambisonics (treat it as stereo).

I tested this implementation https://github.com/ebu/ebu_adm_renderer and it can handle and render zero order ambisonics

It handles zero order ambisonics as HOA (which is technically not correct but it does the same case with first order)

``

    <audioChannelFormat audioChannelFormatID="AC_00041001" audioChannelFormatName="channel_1" typeLabel="0004" 
   typeDefinition="HOA">

    <audioBlockFormat audioBlockFormatID="AB_00041001_00000001">
        <order>0</order>
        <degree>0</degree>
        <normalization>SN3D</normalization>
        <screenRef>0</screenRef>
      </audioBlockFormat>
    </audioChannelFormat>

``