Initial readout of ads variables during iocInit

[redpiramide]

Currently, all output variables are invalid after iocInit because there is no initial readback. If the plc setpoint is not equal 0, this may cause confusion. To not affect performance, there could be an option to read all variables during iocInit if the user wants to.

The difference to #6 is, that also output variables with write-only operation mode should be read once on init. However, in most use cases #6 should be sufficient if all output are configured with readback.

The ISISComputingGroup has added an iocsh function to read single variables (ISISComputingGroup/adsDriver@729b89e) that may also an option.

See the discussion on the EPICS Tech Talk for Reference:
https://epics.anl.gov/tech-talk/2024/msg00522.php