Parse multipole field parameters through getG4Number to honor units

get_field_parameter_double used bare std::stod, which silently truncates
unit expressions: stod("30*deg") returns 30.0 and drops "*deg". So
multipole/dipole parameters written the documented way (vx/vy/vz as
"10*cm", rotation_angle as "30*deg", strength as "2*tesla") were
interpreted in raw internal units, producing fields with the wrong
magnitude, position, and rotation — silently, with no error.

Route the double accessor through gutilities::getG4Number, which parses
unit expressions (minimum_step in gfield_options.cc already uses it).
get_field_parameter_int stays on stoi: its only consumer is pole_number,
a unitless count.

Fixes #107

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: zhaozhiwen

gemc/gfields/gfield.h

@@ -7,6 +7,7 @@
 // gemc
 #include <gemc/gfactory/gfactory.h>
 #include <gemc/gbase/gbase.h>
+#include <gemc/guts/gutilities.h>
 
 
 constexpr const char* GFIELD_LOGGER   = "gfield";
@@ -148,7 +149,7 @@ class GField : public GBase<GField>, public G4MagneticField {
 	 * \param key Map key to retrieve.
 	 * \return Parsed floating-point value.
 	 */
-	double get_field_parameter_double(const std::string& key) { return stod(gfield_definitions.field_parameters[key]); }
+	double get_field_parameter_double(const std::string& key) { return gutilities::getG4Number(gfield_definitions.field_parameters[key]); }
 
 	/**
 	 * \brief Hook for configuring module loggers from options.