[FIX] Resolve session path for subjects with single session

[surchs]

Closes #168

[github-actions]

Pull Request Test Coverage Report for Build 5296115180

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.177%

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Totals
Change from base Build 5286878156:	0.0%
Covered Lines:	482
Relevant Lines:	486

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💛 - Coveralls

[github-actions]

Pull Request Test Coverage Report for Build 5296115180

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.177%

---

Totals
Change from base Build 5286878156:	0.0%
Covered Lines:	482
Relevant Lines:	486

---

💛 - Coveralls

[alyssadai]

Looks good @surchs!

I think we can now actually switch to purely using layout.root to fetch the bids directory path in this util function (get_session_path), which may simplify the function definition/call further. Opened a refactoring iss for this, #171.

[alyssadai]

Given unavailability of PR assignee this week, will merge on their behalf.