Fix bug with surfaces and external atlases

[tsalo]

Closes #1303.

Changes proposed in this pull request

• Don't raise an error in select_atlases if external atlases are provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.89%. Comparing base (16b16f7) to head (2eda078).
Report is 39 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1304   +/-   ##
=======================================
  Coverage   78.88%   78.89%           
=======================================
  Files          59       59           
  Lines        7375     7376    +1     
  Branches      956      956           
=======================================
+ Hits         5818     5819    +1     
  Misses       1266     1266           
  Partials      291      291           

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