Expose SubjectGroup as a column in participant-level results TSV

[alyssadai]

The API already returns subject_group as part of the "subject_data" key in the response object. The query tool should expose this information to the user as part of the participant-level downloads.

This way when users query for healthy controls, there is some explicit evidence (besides/in addition to their Diagnosis being empty) that the matched subjects are indeed healthy controls in our graph.

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[github-actions]

We want to keep our issues up to date and active. This issue hasn't seen any activity in the last 75 days.
We have applied the _flag:stale label to indicate that this issue should be reviewed again.
When you review, please reread the spec and then apply one of these three options:

• prioritize: apply the flag:schedule label to suggest moving this issue into the backlog now
• close: if the issue is no longer relevant, explain why (give others a chance to reply) and then close.
• archive: sometimes an issue has important information or ideas but we won't work on it soon. In this case
  apply the someday label to show that this won't be prioritized. The stalebot will ignore issues with this
  label in the future. Use sparingly!