Before setting SM values for MadGraph, check whether corresponding parameters exist in model.

In MadGraphControl, there is the option to set SM parameters (in particular particle massses) to recommended values. However, in some MadGraph models (e.g. loop_qed_qcd_sm) some masses are removed from the model (e.g. the tau yukawa). Trying to set them to a value leads to a crash in the current setup (and shouldn't be done anyway). This MR checks whether parameters (massses, yukawas, but not decays) are defined before setting them to the SM value.