Setting values that change bit depth image take a long time to return

[GDYendell]

If, for example, photon_energy is changed, the detector returns bit_depth_image in the list of parameters that might have changed and should be fetched. This means the driver automatically fetches bit_depth_image image, which takes a few seconds to calculate on the detector. This has occasionally meant the readback for photon energy does not update for over 5 seconds, causing our GDA to fall over with a timeout.

Perhaps fetchParams could be run in a separate thread. Although, that could mean it is possible to start an acquisition based on a bit depth that is out of date, by setting photon_energy, which would then return straight away, reading the bit depth and opening a file with the matching (wrong) data type and then starting the acquisition. This could be avoided by setting some state when fetching that can be waited on by clients.