Update pipeline: graph now maps IoT devices' MACs to hostnames or other MACs (if...

Update pipeline: graph now maps IoT devices' MACs to hostnames or other MACs (if local communication), and device-specific DNS queries are taken into account.
Changes:
- base_gefx_generator.py updated such that it considers MAC addresses instead of IPs and considers device-specific DNS queries for the IoT devices during graph construction.
- extract_from_tshark.py: updated to also include eth.src and eth.dst.
- parse_dns.py: corrected return value of hostname_for_ip_at_time such that the method simply returns the hostname instead of a tuple of timestamp and hostname.
- updated example json generated by extract_from_tshark.py
- added example gephi file generated by base_gefx_generator.py