Transforms that add variables that aren't used shouldn't trigger re-runs

[gordonwatts]

When we have a variable transform that creates a tuple or a multi-variable item, the cache key should be robust against adding a new item if it doesn't matter.

For example, see how we used LINQToTTree in creating inputs for ski-kit-learn. We added a new variable and everything that did the counting had to re-run.