Add contour lines for a set of qvalues in the plot

[tdelhomme]

The idea is to compute, at each position, for a grid (DP,AO) of n points, the corresponding qvalues if we add the point in the observations.
For each point of the grid, its qvalue will correspond to a transformed p-value computed with pnbinom using estimated parameters of the regression including the point.
This will allow us to plot contour lines for a set of qvalues.
From this, we could also compute, for a specific q-value threshold, the distribution of min_af (=AO/DP in the contour line of the threshold) in function of DP.

[tdelhomme]

This would let the user know what would be the minimum allelic fraction detectable with his threshold.