Allow for easier calls to included data

Hi Matt,

We are using this library intensively. However, one thing that is very cumbersome somehow, is working with relationships and included object.

We are looking for way to expose the relationships included data easier. Something like this

let objectDocument = Document(..)
print(objectDocument.name) // attribute
print(objectDocument.relationships.employee.id) // which already works
print(objectDocument.includedRelation(id: objectDocument.relationships.employee.id).name // which doesn't exist but would be nice to have

And maybe something like this for the entire array?

print(objectDocument.includedRelation(relation: objectDocument.relationships.employee) // would return [Employee]

I have seen the examples of all the caches etc but it results in a lot of boilerplate code and messy code imo. Is something like above possible or is this some limitations we have in Swift currently? I would also love to help with a PR myself if you could give some advice on how to approach this :)

Love to hear your thoughts!

Inizia dagli esempi di Document nell'issue e rivedi gli esempi esistenti di cache per capire come vengono attualmente acceduti le relazioni e gli oggetti inclusi. Definisci come includedRelation debba risolvere un singolo oggetto correlato e un intero array di relazioni, quindi conferma che l'API proposta rimuova il boilerplate descritto senza modificare il comportamento esistente delle relazioni.