Enable playback feature in transmitter apps

[jcfreeman2]

I say "transmitter apps" in the title so as to include one or both of dpdklibs_test_frame_transmitter and dpdklibs_test_transmit_and_receive. The idea would be to have a command line argument like --playback <filename> where <filename> would be a binary file containing ethernet packets that could be extracted from the file and sent out on the wire. Note that a couple of weeks ago I added functions to include/dpdklibs/udp/Utils.hpp (add_file_contents_to_vector and get_ethernet_packets) which should make this task considerably easier. We could also consider updating dpdklibs_test_frame_receiver so that after it opens its currently-existing dpdklibs_test_frame_receiver.dat file it fills it with the ethernet packets it receives.