Be clear about desired vs actual transmission

[DominicOram]

Currently, inside xray centring, we take the transmission specified in the ispyb section of the supplied parameters to mean the desired transmission and set it on the device. We then read this back to find the actual transmission and put that back in the ispyb params object to put in ispyb.

We should instead specify the desired transmission as part of the experiment parameters, then use that to set on the hardware and read the actual back from there to put in ispyb. Note that the removal of the putting the readback into ispyb_params will be done as the last part of #1218. So this issue mostly boils down to move desired_transmission into the experiment params (not in the base class as not used by PinTipCentring)

Acceptance Criteria

• The desired transmission is set in the experiment params
• Plans no longer make reference to ispyb_params for this

[DominicOram]

We think 1 day