Add a helper method for retriving kinetic energy

Author: EinarElen

include/SimCore/Event/SimParticle.h

@@ -63,6 +63,12 @@ class SimParticle {
    */
   double getEnergy() const { return energy_; }
 
+  /**
+   * Get the kinetic energy of this particle [MeV].
+   *
+   * @return The kinetic energy of this particle.
+   */
+  double getKineticEnergy() const { return energy_ - mass_; }
   /**
    * Get the PDG ID of this particle.
    *