Fragmenting of trees added to a fragmented model

[markus1978]

When a containment hierarchy is added to a fragmented model, this hierarchy needs to be analysed for further fragmentation. If a containment hierarchy is build in the absence of a fragmented model, it is not fragmented right away. It must be fragmented if this hierarchy is added to a fragmented model.

At least, we must use the default fragmentation, so there is no absence of a fragmented model.

This should have beed part of the fix for #29