Draft: AGIPD count pixels per gain stage

Requested by Mads (on behalf of MID / an LDC): count how many pixels are in different gain stages so we can monitor that over time. The silly thing is that I already put thresholded gain stages into a full-size array for preview purposes, but:

• We don't send this out (would be terrible waste of bandwidth)
• We don't pass this to addons (API not flexible enough yet)

So in this MR, I've now added an extra hook which sits in a similar place to addons, but gets all the extra buffers that are passed between the device and the kernel runner. This is ugly, but:

• I don't think now is a good time to refactor the device-runner interface again
• Likewise, we could probably make addons more powerful, but
  • maybe not spontaneously (this feature is desired next week I think)
  • maybe this is not an addon, but something we always want, because:
• Saturation monitoring is already a topic - for HED mostly, with Astrid, I put up the Saturation warning aggregator with corresponding addon. That thing warns you if above threshold number of pixels have above threshold value (after correction). Maybe with DET we could decide on something cohesive?