Add missing sigma calculation for pfind output

[pyuxiang]

Once the final time and frequency offset is calculated, we should do one more cross correlation with the modified timestamps and output the significance of the peak (sigma), in line with the output of pfind.c.

This is distinct from the double pfind run on (alice, bob) and (bob, alice) - this belongs to the tests to check for algorithm correctness.

[franciumxzf]

In the previous pfind, we use the sigma to decide whether the peak is strong enough to start tracking. In this current pfind, as we run time_freq multiple times to make sure the frequency result is correct, so we will put aside this sigma calculation aside first.