BUG: Locale-independent itk::StringToDouble/Float; remove NumericLocale

itk::StringToDouble and itk::StringToFloat (itkStringConvert.h) used
std::stod/std::stof, which follow the ambient LC_NUMERIC locale: under a
decimal-comma locale "0.878906" parsed as 0 at every IO call site that
was migrated to these helpers.

Parse with strtod_l/strtof_l against a cached "C" locale, keeping the
itk::ExceptionObject contract. Only overflow throws; ERANGE underflow
yields a valid subnormal or zero and is accepted (std::stod rejected
those valid inputs on some platforms). An unobtainable "C" locale
throws instead of silently parsing under the ambient locale.

Remove the NumericLocale RAII class (never in a release): switching the
thread-local locale via uselocale() is a no-op for the non-_l
std::strtod on macOS 13 and earlier libc, so the class cannot honor its
contract; its last users are gone (NrrdIO is locale-independent by
construction and DCMTKTransformIO now parses DICOM Decimal Strings via
itk::StringToDouble, gaining itk::ExceptionObject wrapping). The
newlocale/_configthreadlocale try_compile checks remain for
itkStringConvert.

Author: hjmjohnson

Modules/Core/Common/CMakeLists.txt

@@ -158,7 +158,7 @@ try_compile(
   ${CMAKE_CURRENT_SOURCE_DIR}/CMake/itkCheckHasSchedGetAffinity.cxx
 )
 
-# Check for thread-local locale functions for NumericLocale
+# Check for thread-safe locale functions (locale-independent parsing in itkStringConvert)
 try_compile(
   ITK_HAS_NEWLOCALE
   ${ITK_BINARY_DIR}

Modules/Core/Common/include/itkNumericLocale.h

@@ -43,6 +43,12 @@ namespace itk
  * `context` describing what was being parsed (e.g.
  * ``"NRRD header field 'sizes'"``) plus the offending input string.
  *
+ * The floating-point helpers parse with an explicit "C" locale
+ * (`strtod_l`-family), so `.` is the decimal separator regardless of
+ * the ambient `LC_NUMERIC` locale — unlike `std::stod`/`std::stof`,
+ * which follow the ambient locale. Underflow to a subnormal or zero is
+ * accepted as a valid value; only overflow throws.
+ *
  * **Fixed-width integer return types** are used (`int32_t`, `int64_t`,
  * `uint32_t`, `uint64_t`) rather than the platform-dependent C++
  * spellings (`int`, `long`, `unsigned long`). The C++ Standard only

Modules/Core/Common/include/itkStringConvert.h

@@ -106,7 +106,6 @@ set(
   itkMultipleLogOutput.cxx
   itkMultiThreaderBase.cxx
   itkNumberToString.cxx
-  itkNumericLocale.cxx
   itkNumericTraits.cxx
   itkNumericTraitsCovariantVectorPixel.cxx
   itkNumericTraitsDiffusionTensor3DPixel.cxx