Membrain-seg should generate binned mrc itself

[stephen-riggs]

Rather than generating all the binned mrc files in the easymode service, we should do the membrain one in that service. This means we can display the 3D visualisation for SXT where easymode does not run.

Most of the diff here is just moving the generate_binned_mrc function into a new file (the function itself is unchanged).

[codecov]

Codecov Report

❌ Patch coverage is 96.87500% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.36%. Comparing base (73061a6) to head (d0f7f3b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #295      +/-   ##
==========================================
+ Coverage   83.06%   83.36%   +0.29%     
==========================================
  Files          76       77       +1     
  Lines        9584     9936     +352     
  Branches     1289     1384      +95     
==========================================
+ Hits         7961     8283     +322     
- Misses       1120     1140      +20     
- Partials      503      513      +10     

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