configure and deliver RAW data alongside imaging

[weatherhead99]

This may be a feature request or simply a documentation request, I didn't see easily how to enable it in current main though.

I would like the ability to configure the raw capture of the archon (i.e. via the RAWSEL, RAWPIXELS etc keywords), and then to have that data read out alongside image data (it's in the same buffer just in a different segment of RAM). Ideally, in FITS saving mode this would be in a separate HDU, and in 0mq mode it would be segmented from the other taplines by some header that indicates it's a RAW capture.

Note I'm not asking to be able to replace image data with raw data, I'm asking to read out both the image and raw from the same buffer (this is perfectly possible in Archon though not documented all that well).

Cheers
DW