v0.6 - Major refactorings to Entity and Relationship class

• Move entity, relationship to utils folder
• Create mixins VisitSubclassesMixin, CreateInstanceMixin, and ExternalComponentMixin
• Refactor External...Entity class using mixin
• Create abstract class GraphComponent and update inheritance for both Entity and Relationship class
• Remove duplicated code
• Fix visit_subclasses() that it should not yield the invoked class itself
• Add dynamically importing all .py files, recursively