Re #7489. Add back mutex per Pete's instructions.

Hoping that this will fix sporadic failures in system (LoadLotsOfFiles
& BASISAutoReduction) & unit (ModeratorTzeroLinear) tests that load
BASIS data.

Code/Mantid/Framework/DataHandling/src/LoadEventNexus.cpp

@@ -284,6 +284,7 @@ class ProcessBankData : public Task
         double tof = static_cast<double>( event_time_of_flight[i] );
         if ((tof >= alg->filter_tof_min) && (tof <= alg->filter_tof_max))
         {
+          alg->m_eventVectorMutex.lock();
           // Handle simulated data if present
           if (have_weight)
           {
@@ -329,6 +330,8 @@ class ProcessBankData : public Task
 
           // Track all the touched wi (only necessary when compressing events, for thread safety)
           if (compress) usedDetIds[detId-m_min_id] = true;
+
+          alg->m_eventVectorMutex.unlock();
         } // valid time-of-flight
 
       } // valid detector IDs