Tree Decomposition without normalization should have empty leaf nodes

[gcharwat]

Currently, during (semi)normalization, empty leaf nodes are added to the tree decomposition. This would also be useful for non-normalized ones, for example when one wants to compute the costs on basis of the child costs. Currently, you can not use
cost(X) :- childCost(Y,Z), ...
directly as childCost/2 is not available in the leaf nodes.