SUBROUTINE QIPVWT(WT) - Extra weight factors to give cross section

This routine calculates the weight factor (WT)

$$W_n= 2\,\pi\,\alpha^2\, \sum\limits_{q^\prime} e_{q^\prime}^2\ x_{\rm Bj}\, x^\prime ,$$ (1)

where $\alpha$ is the electro-magnetic fine-structure constant and $e_{q^\prime}$ is the fractional electro-magnetic charge of the quark $q^\prime$. The sum extends over all quarks and anti-quarks of all $n_f$ flavours.