Separate all model parameters from discretization parameters in input file

[jbreue16]

We have some model parameters that are specified in the discretization subgroup, namely NBOUND, NPARTYPEand PAR_GEOM. They should be moved up by one level, i.e. into the unit_XXX group to achieve a separation of model and discretization parameters, making the interface more consistent/clear.
This should not be a breaking change, i.e. we should read from the old location if the new one does not exist.