Update variable / class naming conventions

[maurei]

Proposal is to normalise several class names and property names.

Related to #317

• RelationshipAttribute.DependentType and .PrincipicalType is not adequate. Replace with something like LeftType vs RightType or HostType vs RelatedType.
• rename ContextEntity to ResourceContext, so that it is consistent with ResourceDefinition and ResourceHooks
• ContextEntity.EntityType --> .ResourceType and current .ResourceType --> .ResourceDefinitionType.
• EntityResourceService --> DefaultResourceService