Simplify stimulus selection

[dmeliza]

The DataSource handling of stimulus names is somewhat clunky. From a UI perspective, the only reason someone should have to supply a list of stimuli is if they want to limit which trials to look at. Otherwise the stimulus names can be inferred from the response files. In fact, things start to break down badly if there are stimuli in the response files that are not loaded. It's a bit slower to read through the pprox files to get a full list of stimuli, but I think it's probably safest and simplest to do it that way.

This came up because tests started failing when I added an option for users to restrict the stimulus list in b839623.