Allow to use a variadic number of pass Ids in discretemodels
Additionally, we do not have to use "-1" as end marker.
This would change the default behavior of classes DGDiscreteModelDefaultWithInsideOutside
and DGDiscreteModelDefault
because "-1" is not allowed anymore as end marker.
So: Make sure to avoid the "-1" end markers!
In dune-fem-dg one can easily use a variadic int parameter instead of passing "-1" to the underlying discrete model.