scripts: test multipath delay and plot with pandas

The test_multipath now measures round-trip-delays using ping, and prints
the results to stdout. The new plotting script (multi_plot.py) parses
the concatenated output from udp_server.py and multi_plot.py and plots
the results using python pandas.
netmix: fix RLNC with sliding window

The new sliding window implementation from kodo requires different ways
to test for partial complete decoding. The decoder now compares the
remote rank with the local rank and with number of decoded symbols.
netmix: update udp_server and udp_client examples

The udp_* examples were outdated and thus needed care. It now uses the
keyword arguments. This process also revealed a few warnings in the
plain_hdr and the inet_sock.