EntityModel does not implement Ctor taking EntityFactoryResolverDelegate

[andyward]

MemoryModel has constructor overloads that take an EntityFactoryResolverDelegate - which allows the EntityFactory to be determined once the header has been read, meaning a single parse.

EsentModel does not provide this overloaded ctor, so the consumer needs to know in advance - and this constraint is then enforced on higher level services in the IModelProviders and IfcStore.

This becomes an issue when models are provided in IO.Streams as not all are seekable.