OCP: AV.io mounts

Clamps for [Epiphan AV.io](https://epiphan.com/products/avio-hd/) video grabber (all variants). This is part of a set of models to build an [Opencast](https://opencast.org) capture agents.

The idea is to have one AV.io mounted using one `avio-mid`, a second one using two `avio-side` and then a power supply using a model like `powersupply` (this model is for one from a DELL mini PC).

Both AV.io's, the power supply and the mini PC were meant to be mounted in a [19" rack adapter](https://thomann.de/de/thon_rackwanne_1he.htm).

All parts are mounted from the bottom using 4mm metric screws. A nut is put over the holes from the side.