Add attribute fan outs to reduce puts required for an acquisition

[GDYendell]

Currently many different PVs have to be put to the same value or synced between Odin and Eiger. Now that we have a single driver we should be able to simplify this a lot. We can couple together the following:

• EF:Acqid, FP:FilePrefix, FP:AcquisitionId, MW:FilePrefix, MW:AcquisitionId
  • This could be FilePrefix (which doesn't set AcquisitionID) and AcquisitionID, which sets everything, in case it needs to be different
• FP:FilePath, MW:Directory
• FP:ProcessFramesPerBlock, EF:BlockSize
• Detector:BitDepthImage, FP:DataDatatype

We could potentially have one top-level "Arm" that arms the detector, gets the bit depth, sets the datatype, starts the file writer and waits for the fan to be ready

Acceptance Criteria

• Fewer puts

[GDYendell]

Any thoughts on this @DominicOram ?

[DominicOram]

This would be great from my perspective, really simplifies things. I guess usual gotchas/requests are:

• The put callback on top level fanout PVs should wait for underlying ones
• Adding this shouldn't make it slower to arm (I can't see how it would but just making the requirement clear)