probe: Added truncated mean averaging capability

[nickweedon]

Added a new samples_trunc_count configuration attribute to the '[probe]' section that will cause the samples_trunc_count number of highest and lowest samples to be removed and then the average of the remaining samples is returned.

This feature is very useful when performing 5 samples or more since it will allow for the 'best of both worlds' with regard to using either median or mean (average) since it gives the converging benefit of mean while still being resistant to outliers like median.
This works particularly well when also combined with samples_tolerance_retries.

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