Support N writers for a single AreaDetector

[jwlodek]

Currently there is a 1-to-1 mapping of ADWriters to AreaDetectors, however, there is no reason why we couldn't have multiple filewriting plugins for a single detector. We're interested in having a setup like this where we use one plugin to save an HDF file with a stack of images, and use another plugin to save the sum of all of those.

Acceptance Criteria

• Can create an AreaDetector with multiple file writers who's state is combined when waiting for the detector to finish.
• The multiple writers can collect different numbers of frames, scaled by a constant (like what we have with exposures_per_event)

[coretl]

So let's say you have a 1000 frame scan, are you expecting:

• A (1000,h,w) dataset from one writer
• A (1, h, w) dataset from another writer that is the sum of 1000 frames
  ?

Or are you expecting it to sum every N instead? Will you use the proc plugin for this?

This seems like the requirement for #1006, but wanting to do have it alongside another HDF file that contains all the frames

[jwlodek]

Ideally I'd like to configure it to sum any N, so I can have 1000 raw frames and 1 summed, or 10 summed of 100 raw each. And yes, essentially this is the same requirement as that other issue, with the extra catch that I'd like to be able to write both from the same detector.

This also encompasses cases where the frames won't be summed, like let's say I'm taking two ROIs and I save two HDF files that are otherwise unrelated that are both 1000, h, w, just different regions of the original image. Meaning that unlike in #1006 the datasets would be of the same first dimension, but potentially different h/w.