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

[David Grellscheid]

>> 2. assert() can be used without "#include <cassert>"
>
> Is that a bad thing? We include it in Rivet/Tools/RivetSTL.hh, along
> with the various "using std::*". User convenience again. The header
> chain needs to be cleaned up (any good tools to help with disentangling
> #include messes?) but that's a separate issue.

Here I agree with David. Since assert() is a macro controlled at compile 
time by NDEBUG, it cannot be safely used in header files anyway. Every 
.cc that needs must include it locally itself.

   David