HepMcParticleLink constructor syntax clean-up.

Avoid using 0 in combination with HepMcParticleLink::IS_EVENTNUM in constructor calls as this is not properly supported. It is more correct to directly use 0 with HepMcParticleLink::IS_POSITION to indicate that the link is to the first GenEvent in the McEventCollection.

(Relates to ATLASSIM-6999)

Generators/GeneratorObjects/GeneratorObjects/HepMcParticleLink.h

@@ -380,8 +380,8 @@ public:
    * @param sg Optional specification of a specific store to reference.
    */
   HepMcParticleLink (const HepMC::ConstGenParticlePtr& p,
-                     uint32_t eventIndex = 0,
-                     PositionFlag positionFlag = IS_EVENTNUM,
+                     uint32_t eventIndex /*= 0 */,
+                     PositionFlag positionFlag /*= IS_EVENTNUM */,
                      IProxyDict* sg = SG::CurrentEventStore::store());