[whahnsr]

Summary

Optimize the single-mask radial distribution calculation for orthogonal
cells.

Profiling showed that essentially all runtime for a representative radial
calculation was concentrated in Action_Radial::calcRDF_singleMask() and
the distance routines it calls.

For orthogonal cells, this change caches the box lengths once per frame and
performs the orthogonal minimum-image distance calculation directly in the
inner pair loop. Other imaging modes continue to use the existing DIST2()
path.

Using std::fabs() for the coordinate differences also allows the compiler
to generate branchless absolute-value operations in this hot loop.

Performance

Test case:

• test/Test_AutoImage/walp_2.strip.dcd
• 11,670 atoms
• 71 frames
• radial DOPC :DOPC out ... 0.25 15.0
• Serial GNU build with -O3

CPU user time:

• Before: 33.28 s
• After: 20.54 s

This is approximately a 38.3% reduction in CPU time, or a 1.62x speedup.

Validation

• All 17 comparisons in test/Test_Radial pass.
• RDF output from the benchmark is byte-for-byte identical to the output
  produced by the original code.
• The modified Action_Radial.cpp was also compiled successfully with
  -fopenmp to verify the OpenMP code path.
• git diff --check reports no whitespace errors.

Non-orthogonal and no-image calculations retain the existing DIST2()
path.

[drroe]

Thanks!