revid: ability to record, encode and send multiple audio sources

[Trek Hopton]

We want the ability to record from multiple microphones at the same time.

The resulting MPEG-TS should contain the multiple streams so that they are in sync with each other, and can be played back/viewed together.

An example use case is having two hydrophones, 100m apart. They are recording for the same period of time at the same site. Analysing both audio recordings together can give us useful information about direction of sound sources.

Revid has only ever dealt with one input source at a time so this may require significant changes to the revid pipeline.