Change inheritance of NetEntity so it is not a NetworkResource any longer

The NetEntity should not be a NetworkResource anymore. The reasoning is that we dont want to force the limited availability concept of the RTM on to implementers. Implementers can then decide on a more suitable concept for that, the for example allows for periodicity. Additionally this change will disallow aggregation of netentities. netentities are supposed to be placed on netelements. these may be aggregated.