Draft: Eiger hardware saving

This allows the eiger to be used in hardware saving without having to mess about manually setting _managed_mode and file_format. lima_base, camera, and saving are all totally decoupled so im not totally happy about where the code is, i'd appreciate feedback on how this could be better organised. The layout is a bit awkward:

eiger = config.get("eiger")
with eiger.camera.hw_saving():
    ascan(..., eiger)

will enable hardware saving for this scan, and put things back again afterwards

The modifications to the acquisition device will make sure that acq_nb_points is a multiple of frames_per_file, which is required by the eiger in order for the scan to actually finish. Here it is not easy to add specialisation for a particular camera, maybe this code can be moved elsewhere?

We also have to make sure that video_active is false as lima cannot run any processing in hardware managed mode.

I fully expect to refactor this! Also not entirely sure how eiger1 specific this is (id13)