fix: use %in% in BenchmarkAggr$subset() rather than ==

[jemus42]

I found that $subset() threw a warning if learner or task was a character length greater 1.
While I wasn't able to create a simple regex to reproduce it, I don't see any downside to using %in% here though, which appears to work just fine in my case.

[jemus42]

Ah nevermind, I see that #36 also addresses this while also changing other parts of the class.