Add Model.WorldPivotData custom getter/setter

[kennethloeffler]

This PR closes #391 by adding a custom getter/setter for Model.WorldPivotData.

I'm a little shaky about what ought to happen when the setter receives a nil. It likely means that the model's PrimaryPart was set to nil elsewhere... I think the safest option is probably to do nothing?

[Dekkonot]

I'm not sure about the PrimaryPart thing. GetPivot works just fine without it, so requiring a PrimaryPart for it feels odd!

On the other hand, I get it and think us being conservative here is probably fine. Roblox doesn't necessarily have a way to convery "optional cframe" from within their engine, so it working regardless may just be an artifact of that.

Going to hesitantly approve this because it works and I understand the reasoning. If it ends up being a mistake, it's not like it's a huge ordeal to correct.

[kennethloeffler]

So, in my dazed confusion I did mess this up a little bit.

Model.WorldPivotData is only unoccupied for a newly created Model. If I set the pivot whatsoever, the property remains occupied regardless of if the model has a PrimaryPart or not. With this in mind, the if instance.PrimaryPart == nil then bit is crap and should be removed.

This also means that when we want the property to be unoccupied (i.e. when the setter receives a nil), we may have to create a new Model instance. I'm not sure if this level of accuracy is necessary, but we should keep it in mind