axis record: add average position field

It could be useful to have an average readback value in the axis record, this may be useful in the case of noisy readbacks. The compress record looks to be able to do such averaging, if we were to set this record to scan every e.g. 0.1 seconds and average the last 10 values, then it would show a 1 second rolling position average.

## acceptance criteria

* configurable averaging mechanism added to axis record