Base/Const/PhysicalConstants: add pi to constants

Base/Const/PhysicalConstants.h

@@ -17,7 +17,11 @@
 #endif // SWIG
 #ifndef BORNAGAIN_BASE_CONST_PHYSICALCONSTANTS_H
 #define BORNAGAIN_BASE_CONST_PHYSICALCONSTANTS_H
+#ifdef BA_CPP20
 
+#include <numbers>
+
+#endif // BA_CPP20
 //! Physical constants.
 
 namespace PhysConsts {
@@ -30,6 +34,11 @@ constexpr double r_e = 2.8179403262e-15;   //!< Thomson scattering length (\f$ r
 constexpr double gamma_n = 1.91304272;     //!< \f$\gamma\f$ factor for neutron magnetic moment,
                                            //!< \f$\mu_n = \gamma \cdot \mu_N\f$
 constexpr double g_factor_n = -3.82608545; //!< neutron g-factor
+#ifdef BA_CPP20
+constexpr double pi = std::numbers::pi;
+#else
+constexpr double pi = 3.141592653589793238462643383279502884L; // OEIS: A000796
+#endif // BA_CPP20
 
 } // namespace PhysConsts