Add netConnector to allows splitting of infrastructure models

In the course of railML 3.3 development we discussed splitting and merging of infrastructure. Means that a part of the infrastructure description is transmitted somewhere else and later reattached. This is done using a netConnector and a netConnectorRelation. A netConnector is a point-shaped netElement subclass that can aggregate with other netConnectors (but not with other netElements). This way the split of infrastructures can be expressed on any level of the infrastructure (meso, macro). With a netConnectorRelation the netConnector can be connected with netElement. After the netConnector appears in both parts of the split infrastructure. A netConnector needs an identity by which it later can be used for merging the parts together again.