EventContext: make constructor explicit

We do not want to implicitly convert an integer to an EventContext as this would almost certainly result in an invalid event context.

We just had a case in ATLAS where something like the following code compiled fine and only at runtime we got an invalid EventContext error:

void foo(const EventContext& ctx);

int context; // variable incidentally named context that has nothing to do with an `EventContext`
foo(context);

Thanks to @bstanisl for pointing this out. cc @smh

GaudiKernel/include/GaudiKernel/EventContext.h

@@ -41,8 +41,8 @@ public:
 
   EventContext() = default;
 
-  EventContext( const ContextEvt_t e, const ContextID_t s = INVALID_CONTEXT_ID,
-                const ContextID_t subSlot = INVALID_CONTEXT_ID )
+  explicit EventContext( const ContextEvt_t e, const ContextID_t s = INVALID_CONTEXT_ID,
+                         const ContextID_t subSlot = INVALID_CONTEXT_ID )
       : m_evt_num( e ), m_evt_slot( s ), m_sub_slot( subSlot ) {
     m_valid = ( e != INVALID_CONTEXT_EVT && s != INVALID_CONTEXT_ID );
   }