ENH: Add itkVirtual/itkFinal/itkNonVirtual macro variants

[hjmjohnson]

Add itkVirtual/itkFinal/itkNonVirtual variants for all itkSet*/itkGet* macros. Legacy macros unchanged — zero behavioral/ABI change. Silences 611 -Wunnecessary-virtual-specifier warnings in ANTs (issue #6232, PR #2785).

Backward-compatibility guarantee

100 % backward-compatible. Every legacy macro name continues to expand to the virtual variant — byte-identical generated code, identical vtable layout, identical ABI. No deprecation gate, no ITK_FUTURE_LEGACY_REMOVE change, no symbol churn, no API removed/renamed/relocated. Downstream code that never types itkVirtual* / itkFinal* / itkNonVirtual* sees no behavioral or compilation difference. The PR only adds new optional spellings; no existing class is migrated.

What this PR does

For each member-function macro family in itkMacro.h:

1. An internal *Impl helper parametrized by (virtualKeyword, finalKeyword) — the DRY shared body.
2. Three public variants:
   • itkVirtual*Macro → virtual (matches today's behavior)
   • itkFinal*Macro → final (seals further overriding)
   • itkNonVirtual*Macro → no virtual-specifier (new methods in final leaf classes)
3. The legacy spelling, redefined to expand to the itkVirtual*Macro variant.

No itkOverride* variants are provided: re-declaring an inherited Set/Get in a subclass with the exact same macro-generated body just duplicates the base-class implementation. If an override is genuinely needed (different behavior, extra validation, side effects), write the function by hand. The pre-existing itkOverrideGetNameOfClassMacro is unchanged.

Minimal non-breaking foundation for PR #2785 and issue #6232.

Why this matters — concrete ANTs evidence

ANTs has shipped final leaf classes (command-line option types, filter wrappers, statistical containers, image-curvature analyzers, etc.) since at least 2019. For that entire period the legacy macros offered no mechanism to silence Clang's -Wunnecessary-virtual-specifier on the resulting virtual accessors — the only workaround was abandoning the macros and losing itkDebugMacro / Modified() / float-equal-suppression standardization.

Clean ANTs master build against this PR:

• Baseline (legacy macros): 626 warnings across 14 final-marked headers.
• After mechanically substituting itkNonVirtual* at 167 call sites in those 14 headers: 15 remaining (residue is hand-written virtual keywords, not macro-generated).
• Net: 611 warnings silenced — 97.6 % — by call-site spelling alone. No change to function bodies, signatures, or behavior.

Design: the DRY pattern

// One body, three variants, one legacy alias — per family.

#define itkSetMacroImpl(virtualKeyword, finalKeyword, name, type) \
  virtualKeyword void Set##name(type _arg) finalKeyword           \
  {                                                               \
    itkDebugMacro("setting " #name " to " << _arg);               \
    /* ...identical body to original itkSetMacro... */            \
  }                                                               \
  ITK_MACROEND_NOOP_STATEMENT

#define itkVirtualSetMacro(name, type)    itkSetMacroImpl(virtual, , name, type)
#define itkFinalSetMacro(name, type)      itkSetMacroImpl(, final, name, type)
#define itkNonVirtualSetMacro(name, type) itkSetMacroImpl(, , name, type)
#define itkSetMacro(name, type)           itkSetMacroImpl(virtual, , name, type)  // legacy

At most one of virtualKeyword and finalKeyword is non-empty; itkNonVirtual* passes both empty.

Local verification

• pre-commit run --all-files passes at the pushed HEAD.
• ITKCommon and module header tests build clean.
• ANTs master build against this PR yields the 626→15 warning delta above.

[hjmjohnson]

@greptile review