Remove NetworkResources that do not belong to any NetworkLevel

In RTM 1.4 a Network refers to a collection of NetworkResource instances and a collection of NetworkLevels, which themselves have a collection of NetworkResources. This means it is possible to specify NetworkResources that do not belong to any NetworkLevel. Experience with railML has shown that this is not necessary and rather introduces an additional complexity that serves no purpose. The collection of NetworkResources of the Network should be removed.