Add Indexed access support to RealmResults

[AndyDentFree]

From the SO questionRealm dotnet: get element by index from RealmResults we need a way to provide indexed access.

After some discussion, we will first add the LINQ ElementAt as seen in LINQ source and see if that satisfies the OP's needs.

[AndyDentFree]

Expressions like this in SimpleLINQtests.CreateList should work without the ToList

 var s0 = _realm.All<Person>().Where(p => p.Score == 42.42f),ToList();
 Assert.That(s0 [0].Score, Is.EqualTo(42.42f));

[kristiandupont]

This was resolved with #802.