Allow this two-stream example to run with any combination of devices (like its C and python counterparts).
Allow this two-stream example to run with any combination of devices (like
its C and python counterparts).

Also, change polar plot labelling logic to be identical with example 3. With
this change this 14th C++ example produces identical results with the 14th C
and python examples.

svn path=/trunk/; revision=5313

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot