Improve the algorithm in LFN measurement with SR810

Now, for time constant of 300ms, the criterion for final reading on each frequency is determined by

1. at least 15% of full sensitivity range
2. standard deviation of 12 readings with 1s time interval is no bigger than 1% of full sensitivity range
3. Do a linear regression on 12 readings, the slope (unit of A/s or V/s) is no bigger than 0.5% of the mean. (newly added)