Smartem workflow fixes and single-gs tiles

[stephen-riggs]

• Should insert atlas jpg not mrc
• Missing register and object unpack
• Handle case of single grid square on a tile, which gives a dict rather than a list

[codecov]

Codecov Report

❌ Patch coverage is 35.71429% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.92%. Comparing base (afc8fe9) to head (a72e45b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #890      +/-   ##
==========================================
- Coverage   54.95%   54.92%   -0.04%     
==========================================
  Files         103      103              
  Lines       11505    11513       +8     
  Branches     1526     1530       +4     
==========================================
  Hits         6323     6323              
- Misses       4841     4848       +7     
- Partials      341      342       +1     

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