[entitynetwork] restructure networks and builders
Instead of storing both a list and a map, we now only store maps of entity/subdomain fragments -> unique identifiers in order to reduce the memory overhead. Moreover, the public interface of the entity networks is changed such that one can no longer get fragments of a sub-domain with a specific id as the overall map already contains this information. This can be added in the future as an external function to extract data of a specific subdomain from the network. The builder classes are also restructured to no longer store unneccesary data. Only the public interfaces of entity networks have changed, and due to the small user-base, and as that interface is currently only used internally by the writer (which has been modified), we don't guarantee backwards compatibility here.