Fix race condition in Unit.cpp Refs #5584

The static quick conversion map mutated within a parallel loop, see
AnvredCorrection, so it needs to be protected by a critical block

Code/Mantid/Framework/Kernel/src/Unit.cpp

@@ -2,6 +2,7 @@
 // Includes
 //----------------------------------------------------------------------
 #include "MantidKernel/Unit.h"
+#include "MantidKernel/MultiThreaded.h"
 #include "MantidKernel/PhysicalConstants.h"
 #include "MantidKernel/UnitFactory.h"
 #include <cmath>
@@ -78,8 +79,12 @@ Unit::ConversionsMap Unit::s_conversionFactors = Unit::ConversionsMap();
 void Unit::addConversion(std::string to, const double& factor, const double& power) const
 {
   std::transform(to.begin(), to.end(), to.begin(), toupper);
-  // Add the conversion to the map (does nothing if it's already there)
-  s_conversionFactors[unitID()][to] = std::make_pair(factor,power);
+  // If this happens in a parallel loop the static map needs protecting
+  PARALLEL_CRITICAL(Unit_addConversion)
+  {
+    // Add the conversion to the map (does nothing if it's already there)
+    s_conversionFactors[unitID()][to] = std::make_pair(factor,power);
+  }
 }
 
 //---------------------------------------------------------------------------------------