Optimise reading of parameters in LVM post-processing

[albireox]

Currently we read all the necessary parameters for the LVM headers (pressure, temperatures, hartmann door positions) after the exposure has been read, in the delegate post-process. This is moderately slow (maybe a couple seconds). There are two options to optimise it:

• Make the delegate aware of the LVM status keywords and cache them. Then use the last seen values when building the header. This would require running archon lvm status on a certain interval (shorter than a readout).
• Execute the post_process() method during readout, asynchronously. This would require some changes to the general delegate and the basic header may also need to be created during readout.