Standardize chyron and slate columns.

[shel-ho]

In the newshour-chyron gold csv's, the column is scene-label while in the january-slate gold csv's, the column is scene-type. For the purpose of evaluation, these should have the same column title. Since there are plans to also evaluate language identification which does not label scenes, I propose a more general label like label or something similar.

[keighrim]

Also related to #106 . I used scene-type as the column name for other tasks (understanding-xxx, scene-recognition, ...) as I mentioned in that issue comment, but our convention document clearly says scene-label is the right one. One issue now is renaming scene-subtype column name in the SR task.

[keighrim]

Possible solutions;

• update all tasks with scene-type to scene-label and fix their task readme, then
  • rename scene-subtype to scene-sublabel (but sublabel sounds a bit awkward...?), OR
  • keep scene-subtype (although its clear relation to "scene type"), OR
  • rename scene-subtype to scene-subtype-label (a bit too long, and still not clear why we use scene-label for its counterpart instead of scene-type-label )
• , OR update the convention and promote scene-type as the standard name. But I agree to Shel that "label" is more generic and suitable term here.