Caching IndexedSource

[garrettwrong]

@j-c-c found that the cache method for indexed source may not be functioning as expected.

This was found by adding .cache() into the current pipeline tutorial before into the class averages and observing that the images were being generated multiple times. I think this began after the as_copy updates, and went unnoticed for a while.

I'll try to add a smaller script/test that demonstrates the issue.