Re-bin method for PG-statistics

[wcxve]

It is well known that profile likelihood can be biased. For W-statistics, numerical experiments showed re-binning can avoid this bias (see e.g., Giacomo's blog and Johannes's review).

For PG-statistics, a certain criteria, $\hat B/\hat\sigma_B^2 \ge \alpha(1-N/S)$, should be met so that the background will not be negative. If the criteria is not met, then the background will be truncated at zero, which introduces a similar bias like W-statistics. We should also conduct a numerical experiment to see how to avoid the bias.

[wcxve]

Added group='bsig' option in Data initialization.