Fix inconsistent parameter naming #108
Description
For example, in ModelMeaslesQuarantine the transmission_rate parameter is named "Transmission rate" internally (e.g., in the output of summary() or when calling set_param(), but in the ModelSEIRMixing that parameter is named "Prob. Transmission" instead. We should unify these internal names so that a user always sees the same name for the same parameter across different models.