re-ordered class for IcmpPacket

re-ordered class for IcmpPacket

old: IcmpPacket has and IpvXHeader, an IcmpvXHeader which varies according to ICMP message type and also contains part of Icmp message data, and an IcmpData which is a string;
new: IcmpPacket has an IpHeader, an IcmpHeader which represents only the common first 4 byte of ICMP v4/v6, and IcmpData which is either a IcmpEchoData for echo request/reply or a IcmpDestinationUnreachableData

tried to abstract away as much as possible (for a lay programmer like me) the difference between v4 and v6

This code now compiles, need to see whether it does anything usefull

Tests not compiled/run yet