Add an nSigma option to the MET signature dict
We want to try out different values for nSigma for the various pufit algorithms. This is the first step: to let the configuration know about the option. The strings in the trigger names will be the nSigma value multiplied by 10, so HLT_xeXX_tcpufit_sig40 would have σ=4. The next steps will be picking triggers and adding them to the menu, along with the relevant containers to the EDM.