Don't initialize a reference from a temporary in molecular_dynamics_cells

[LegalizeAdulthood]

Fixes #99

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.35%. Comparing base (0ce2771) to head (1489e27).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #100   +/-   ##
========================================
  Coverage    99.35%   99.35%           
========================================
  Files          168      168           
  Lines         6833     6833           
========================================
  Hits          6789     6789           
  Misses          44       44           

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