Allow study-level data access at contrast level

[tsalo]

Add levels argument to Dataset._load_data, in order to load data from the study level when appropriate (e.g., for metadata and texts, but not for images).

Metadata (and texts, to some extent) can exist over multiple levels for a study. We need to be able to access study-level data at the contrast level.

Stems from #179 and possibly related to #218.

[tsalo]

I believe the NIMADS solution was to simply duplicate all study-level metadata at the analysis level. If so, we can close this. @jdkent is that correct?