Model: Separate Channels from Epochs

[jschaeff]

As for stationXML v1.x, epochs in a channel are defined as new channels.

What about modeling in the metadata that one channel has one or more epochs ? Like this 👍

<Channel>
    <Epoch starttime="" enddime="">
    </Epoch>
</Channel>

We could put everything defined in a channel v1.x inside of the Epoch element.

This would be more consistent with the classical data model and simplify metadata exploration.