Allow AnnonatedTopologyReader::AssignIpv4Addresses() have custom mask

[RobinHood3082]

The only current implementation is this:

void
AnnotatedTopologyReader::AssignIpv4Addresses(Ipv4Address base)
{
  Ipv4AddressHelper address(base, Ipv4Mask("/24"));

  BOOST_FOREACH (const Link& link, m_linksList) {
    address.Assign(NetDeviceContainer(link.GetFromNetDevice(), link.GetToNetDevice()));

    base = Ipv4Address(base.Get() + 256);
    address.SetBase(base, Ipv4Mask("/24"));
  }
}

• Since mask is set to /24, it only allows upto 256 Ipv4Address (es)
• Overloading this function might give more flexibility
• Allow setting custom Ipv4mask