Minor bug fixes in CLEM workflow when registering grid squares

[tieneupin]

Pillow uses banker's rounding (round .5 to the nearest even number) internally, so the thumbnail size calculation logic (we are predicting it instead of looking for a file that might not yet be created) has been updated to include rounding when calculating the final image sizes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.89%. Comparing base (9944767) to head (5a362d7).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #774   +/-   ##
=======================================
  Coverage   48.89%   48.89%           
=======================================
  Files          94       94           
  Lines       10008    10008           
  Branches     1326     1326           
=======================================
  Hits         4893     4893           
  Misses       4860     4860           
  Partials      255      255           

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