adm builder: multi-channel item creation and split format from content

[tomjnixon]

A few related changes which make building ADM files with multi-channel streams much more pleasant when using ear.fileio.adm.builder.Builder:

• split MonoItem into format and content parts, Format and Item, to allow formats to be used more than once

  • can now use create_format_mono to create a format, and create_item_mono_from_format to use it

  • in the future we can add methods to extract a Format object from common definitions

  • to keep the API the same, properties are added to Item to access the format attributes which used to live directly in MonoItem

• Format and Item can hold multi-channel formats and contents

  • added multi-channel functions:

    • create_item_multichannel

    • create_format_multichannel

    • create_item_multichannel_from_format

  • added singular properties like track_uid which gets the single audioTrackUID out of track_uids

• added HOA helpers:

  • create_format_hoa

  • create_item_hoa

• added tests

[tomjnixon]

to do:

• [x] HOA utilities should pass on any keyword arguments to AudioBlockFormatHoa, to make nfcRefDist and screenRef work too
• [x] should use this in ambix_to_bwf