added mass density type

Framework/Units/PhysicalUnits.h

@@ -51,6 +51,15 @@ namespace corsika::units::si {
       phys::units::quantity<phys::units::electric_charge_d, double>;
   using EnergyType = phys::units::quantity<phys::units::energy_d, double>;
   using MassType = phys::units::quantity<phys::units::mass_d, double>;
+  using MassDensityType = phys::units::quantity<phys::units::mass_density_d, double>;
+
+  // defining momentum you suckers
+  // dimensions, i.e. composition in base SI dimensions
+  using momentum_d                     = phys::units::dimensions< 1, 1, -1 >;
+  // defining the unit of momentum, so far newton-meter, maybe go to HEP?
+  constexpr phys::units::quantity< momentum_d > newton_second   { meter * kilogram / second };
+  // defining the type
+
   using MomentumType = phys::units::quantity<momentum_d, double>;
   using CrossSectionType = phys::units::quantity<sigma_d, double>;