List Contents

[JamesWiresmith]

Need to be able to query the available groups and channels in the file.

Will need at least two calls:

list_groups() -> Groups
list_channels_in_group(group) -> Channels

Maybe we could list entire contents too:

list_channels() -> ChannelsWithGroupInfo

[JamesWiresmith]

For the channels in a group, if we use a btree we can nearly use the range to get just those of interest. See https://www.thecodedmessage.com/posts/prefix-ranges/