3a_periodicInit.gms will fail with multiple model definitions
There has been an original attempt to allow Backbone loop over multiple models. However, this is broken and would require lot of work to fix. Related to this, there is a problem in 3a_periodicInit.gms in the way it loops through models - the settings from the last defined model are used. This is not a problem if only one model type has been defined. We are not aiming to support iteration between multiple model types any time soon, but in the meanwhile we should halt the model if it tries to use more than one model type.