Merge branch 'fix/loki-thread' into 'master'

Fix Loki initialisation in Extrapolate for threading.

This MR fixes a bug in Extrapolate class with Loki initialisation.

See merge request !492

solvers/IncNavierStokesSolver/EquationSystems/Extrapolate.cpp

@@ -49,7 +49,8 @@ namespace Nektar
     {
         typedef Loki::SingletonHolder<ExtrapolateFactory,
                                       Loki::CreateUsingNew,
-                                      Loki::NoDestroy > Type;
+                                      Loki::NoDestroy,
+                                      Loki::SingleThreaded > Type;
         return Type::Instance();
     }