fix: count non-adjacent atom types

[galjos]

Summary

• Add utilities::getUniqueElements for sorted unique vector values.
• Use the helper in Molecule and MoleculeType atom-type counting.
• Keep the non-adjacent duplicate regression and add direct helper coverage.

Root cause

std::ranges::unique only compacts consecutive equivalent elements. Without sorting first, non-adjacent duplicate atom types were counted as separate atom types.

Validation

• ctest --test-dir build-pr271-static -R '^(testMolecule|testCollectionUtilities)$' --output-on-failure

Reference

• https://en.cppreference.com/w/cpp/algorithm/ranges/unique

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.26%. Comparing base (56d4cb3) to head (8d4720d).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #271   +/-   ##
=======================================
  Coverage   88.26%   88.26%           
=======================================
  Files         284      285    +1     
  Lines       10964    10966    +2     
  Branches     3416     3414    -2     
=======================================
+ Hits         9677     9679    +2     
  Misses       1247     1247           
  Partials       40       40           

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[pq-perf-bot]

⚡ Performance (instruction count) — ✅ no regressions

per-benchmark breakdown

benchmark	base Ir	PR Ir	Δ
bondedForces	41.46M	41.46M	-0.00%	✅
boxTransforms	10.67M	10.67M	+0.00%	✅
constraints	11.38M	11.38M	-0.00%	✅
coulombKernel	5.80M	5.80M	+0.00%	✅
forceKernel	15.33M	15.33M	-0.00%	✅
integrator	32.17M	32.17M	-0.00%	✅
kinetics	9.53M	9.53M	+0.00%	✅
linearAlgebra	2.08M	2.08M	+0.00%	✅
nonCoulombPairs	5.48M	5.48M	-0.00%	✅
shiftVector	5.81M	5.81M	+0.00%	✅
virial	11.63M	11.63M	+0.00%	✅

_{Deterministic callgrind instruction counts vs the base branch; gated at ±2%. Not wall-clock.}

[97gamjak]

ibwould lile to have this generalized with a getuniqueelements or smth similar

[97gamjak]

see other comment