core/affectanalyser.cpp: safer p-value estimation, taking into account the central region
As detected by @mikael-s in the parent commit, the region between V and J affectations was not taken into account in the p-values, yielding erroneous segmentations when this region was very large. Now this region is counted *both* for the computation of left and right p-values, solving the bug of the parent commit. This could be sometimes over-conservative : are we counting things twice ? In regular situations, the answer is no, as the p-values are eventually computed by getProbabilityAtLeastOrAbove in kmerstore.h, that takes into account the length of the seed. A more exact option could have been to use something like (first_pos_max + last_pos_max / 2) + getS()/2, but it would raise symmetry problems. The selected option should anyway improve the estimation in most of the cases.
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