This should allow us to have selections with systematic variations and read them as part of the selection accessors.
As part of this I had to shift the SysDecorationHandle to use a standalone messaging namespace (ANA_MESSAGE_HEADER+ANA_MESSAGE_SOURCE) rather than using AsgMessagingForward. It would be possible to do this with the older class, but would make a lot of the calls around the selection read handle a lot uglier.
I also forgot to turn my code formatter off so there are quite a lot of changes which are only formatting. On balance I prefer to keep these but if makes the review too hard I can also try and revert the changes which are pure formatting.
Note that this only changes the read handle class, not any of the places that use it - they also need to be updated to pass in the correct systematic and to call preExecute where necessary. If it's preferred I can also do that in this MR.