Initialize pvalue attribute name.

def _init_pval_name(self, **kws):
        """Initialize pvalue attribute name."""
        if 'pval_name' in kws:
            return kws['pval_name']
        # If go2res contains GO Terms
        if self.is_goterm:
            return "p_{M}".format(M=next(iter(self.go2res.values())).get_method_name())
        # If go2res contains GO namedtuples
        for fld in next(iter(self.go2res.values()))._fields:
            if fld[:2] == 'p_' and fld != 'p_uncorrected':
                return fld