Tick should return Grade object

[Vichy97]

Ticks should use a Grade object instead of a string for grade. This should have all fields populated by the server when requesting a tick object.

[vnugent]

Indirectly addressed by PR #433

[Vichy97]

@vnugent I don't think that PR quite addresses this. It's true, that the climb has a Grade object, but I was planning on allowing the user to specify their proposed grade when ticking (similar to what you can do on MP). I don't think the climb object should be replacement for the existing grade field