Allow for lazy getting of To-One relations

Given the following model

```ts
@Model()
export class B extends Resource {
	@Field()
	public id: string;
 }
@Model()
export class A extends Resource {
	@Field()
	public id: string;
	@ToOne()
	public toOneB: ToOneRelation<A,B>;
 }
```

Allow for
```ts
a = new A();
await a.toOneB.fetch();
```

