Fix for no borders in multi-materials sims

[RemDelaporteMathurin]

Proposed changes

Fixes #911

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Black formatted
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.55%. Comparing base (a82f0d2) to head (61ad5aa).
Report is 95 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #927   +/-   ##
=======================================
  Coverage   99.55%   99.55%           
=======================================
  Files          60       60           
  Lines        2717     2719    +2     
=======================================
+ Hits         2705     2707    +2     
  Misses         12       12           

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[KulaginVladimir]

LGTM