[Common] Add eta and eta' to PhysicsConstants.h

- Add Eta and EtaPrime meson to the PhysicsConstants.h with PDG value and mass

Author: mhemmer-cern

Common/Constants/include/CommonConstants/PhysicsConstants.h

@@ -31,6 +31,8 @@ namespace o2::constants::physics
 /// \note Follow kCamelCase naming convention
 /// \link https://root.cern/doc/master/TPDGCode_8h.html
 enum Pdg {
+  kEta = 221,
+  kEtaPrime = 331,
   kB0 = 511,
   kB0Bar = -511,
   kBPlus = 521,
@@ -93,6 +95,8 @@ enum Pdg {
 };
 
 /// \brief Declarations of masses for additional particles
+constexpr double MassEta = 0.547862;
+constexpr double MassEtaPrime = 0.95778;
 constexpr double MassB0 = 5.27966;
 constexpr double MassB0Bar = 5.27966;
 constexpr double MassBPlus = 5.27934;