refactor: make virial a pure static class

[97gamjak]

closes #486

[pq-perf-bot]

⚡ Performance (instruction count) — 🎉 improvement: virial -12.79%

per-benchmark breakdown

benchmark	base Ir	PR Ir	Δ
bondedForces	38.24M	38.24M	-0.00%	✅
boxTransforms	10.67M	10.67M	+0.00%	✅
constraints	10.92M	10.91M	-0.04%	✅
coulombKernel	5.80M	5.80M	+0.00%	✅
forceKernel	14.16M	14.16M	-0.00%	✅
integrator	32.44M	32.44M	-0.00%	✅
interWater	1681.44M	1681.40M	-0.00%	✅
kinetics	8.55M	8.55M	-0.01%	✅
linearAlgebra	2.08M	2.08M	+0.00%	✅
nonCoulombPairs	5.48M	5.48M	+0.00%	✅
shiftVector	5.81M	5.81M	+0.00%	✅
virial	10.44M	9.10M	-12.79%	🎉

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

[codecov]

Codecov Report

❌ Patch coverage is 76.66667% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.19%. Comparing base (1ace0e9) to head (e4698c6).
⚠️ Report is 14 commits behind head on dev.

Files with missing lines	Patch %	Lines
src/virial/virial.cpp	75.86%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #490      +/-   ##
==========================================
+ Coverage   91.13%   91.19%   +0.05%     
==========================================
  Files         319      316       -3     
  Lines       14534    14515      -19     
  Branches     4817     4810       -7     
==========================================
- Hits        13246    13237       -9     
+ Misses       1202     1192      -10     
  Partials       86       86              

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Approval dismissed: new commits contain changes beyond a merge-up from dev.

[galjos]

@97gamjak

⚡ Performance (instruction count) — 🎉 improvement: virial -12.78%

because you removed virial timing instrumentation. Should be fix the perf bench to account for this?

[ape33]

use a range-based for loop here