Add GsmtapReceiver — receive GSMTAP-SIM over UDP

UDP socket on port 4729, parses GSMTAP header, validates
GSMTAP_TYPE_SIM, returns (sub_type, data). Compatible with
simtrace2-sniff and pySim's GsmtapApduSource.

simtrace2_pysniff/__init__.py

@@ -2,7 +2,7 @@
 
 from .device import SniffSession, DeviceDisconnected, find_sniffer_device
 from .protocol import parse_message, SniffMessage
-from .gsmtap import GsmtapSender
+from .gsmtap import GsmtapSender, GsmtapReceiver
 from .pcap import PcapWriter
 from .dump import FileDumper, format_message
 
@@ -14,6 +14,7 @@ __all__ = [
     'parse_message',
     'SniffMessage',
     'GsmtapSender',
+    'GsmtapReceiver',
     'PcapWriter',
     'FileDumper',
     'format_message',