Fastjet contrib interface changes

take changes from jet met PR (but only those needed for compilation/running)
Was this confirmed to be thread-safe (or even event-by-event safe, having no past call memory).
There are quite a few mutables in the official version of the code 
http://fastjet.hepforge.org/svn/contrib/contribs/Nsubjettiness/tags/2.1.0/Njettiness.hh