[Rivet] Bug: namespace and include pollution in Rivet namespace (minor)

[David Bjergaard]

Hi,

I know that there are some irregular design considerations to make Rivet
more friendly to non-C++ experts, however these two quirks have been
bothering me, so I'm reporting them.  I understand if nothing is done to
change them, I just want them off my conscience:
1. Rivet namespace includes 'namespace std' (or at least std::cout and
   std::endl)
2. assert() can be used without "#include <cassert>"


    Dave