This changes the interface of the function but I don't want to replicate
that function in all the filters that I apply on top of the parsed data.
My usecase is smth like this:
    for filename in sys.argv[1:]:
        for packet in ifilter(filter, parsefile(filename)):
            print(packet)

I was hesitating to add this, because it's a calculation that is not
necessarily needed by the user of pgpdump. But then again, that is the
case for nearly every data computed by pgpdump. It'd probably be smart
to make all fields needing calculation a lazy Property() and update the
fields after calculation.

I introduced a NameError at some stage:
  File "/home/muelli/vcs/python-pgpdump/pgpdump/__main__.py", line 13, in parsefile
    for packet in data.packets():
  File "/home/muelli/vcs/python-pgpdump/pgpdump/data.py", line 34, in packets
    total_length, packet = construct_packet(self.data, offset)
  File "/home/muelli/vcs/python-pgpdump/pgpdump/packet.py", line 590, in construct_packet
    packet = PacketType(tag, name, new, packet_data)
  File "/home/muelli/vcs/python-pgpdump/pgpdump/packet.py", line 148, in __init__
    super(SignaturePacket, self).__init__(*args, **kwargs)
  File "/home/muelli/vcs/python-pgpdump/pgpdump/packet.py", line 22, in __init__
    self.parse()
  File "/home/muelli/vcs/python-pgpdump/pgpdump/packet.py", line 166, in parse
    "%02x" % (offset, data[offset]))
NameError: global name 'data' is not defined

It should have been self.data of course.