Calculates the relative delay (d1-d2) for a pair of antennas in ns.

def calcdelay(self, ant1, ant2, skyfreq, pol):
        """ Calculates the relative delay (d1-d2) for a pair of antennas in ns.
        """

        select = self.select[n.where( (self.skyfreq[self.select] == skyfreq) & (self.polarization[self.select] == pol) )[0]]

        ind1 = n.where(ant1 == self.antnum[select])
        ind2 = n.where(ant2 == self.antnum[select])
        d1 = self.delay[select][ind1]
        d2 = self.delay[select][ind2]
        if len(d1-d2) > 0:
            return d1-d2
        else:
            return n.array([0])