Add support for unconditional fit only
The current ActionCalculateSignificance is currently used to run the main fit (i.e. produce the main results). However, there are already now cases where the significance is not interesting (or the conditional fit is already bound to fail for numerical reasons). In these cases we should offer an option to skip the conditional (µ=0) fit.
While this is somewhat against the name of the action we should still simply implement this switch there in favor of avoiding code duplication.