Add n bjet observable

Closes #4 (closed)

Only caveat: we get pretty_name: '$N_{\mathrm{b}}^{M}$ in the config whereas we would just like pretty_name: '$N_{\mathrm{b}}$ for the unfolded observable. But let's just move on, worst case we fix it by hand in the config (unless you see an easy way of getting that right from the config writer)...