changed logic of icmp package read/write and operator>> / operator<<

changed logic of icmp package read/write and operator>> / operator<<

* read and write are implemented on their own, >> and << call them
  (not the other way round as before)
* read returns not only a bool but a more informative flag
* moved operator>> and << from Icmpv4/6packet to base class IcmpPacket
  since they are identical