[Rivet] compile error and patch: OSX 10.6.8 or more std C++ compiler

[David Grellscheid]

I deliberately only answered to the list, it's a discussion we need to
have internally first of all.

The code at the moment says this in the comments:

> /// @note This question is valid in MC, but may not be answerable
> /// experimentally -- use this function with care when replicating
> /// experimental analyses!

At least, we should prefix such functions with "unsafe_" or something
like that to provide a little stumbling block when you come across it.

  David