Closes #104. Memory error crashing nds2_channel_source.
The code make a safer command generator for nds2 has an error in the source list/data commands. It writes data to 'cmd' when it should write to 'cmd_txt', overflowing 'cmd'.
This fixes that code, adds a test that basic usage of daq_recv_source_data and daq_recv_source_list work.