Extend the implementation of Entity concept

[SlavaVedernikov]

Context

At the moment there is a little bit of support for Entity in the framework (see C4InterFlow.Structures.Entity).

• There is support for Composition via ComposedOfMany and ComposedOfOne properties and Inheritance via Extends property.
• Entity can be associated with Container by being added under Entities object.
• Entity can also be associated with Interface by being added as its Input and/or Output

Requirement

• Allow Entity to be added to Namespace under 'Entities' object
• Allow Entity to be added to SoftwareSystem under 'Entities' object
• Add 'Realises' property to 'Entity' so that lower level 'Entity' (e.g. at Container level ) can be associated with a higher level Entity (e.g. as Namespace level)