Average is written in .vtu

The average of the species properties can be written now in .vtu format.
No .pvd file is provided as no time series is generated.

Still to do:
  Read a .vtu mesh.
  Improve gmsh format to use more common functions.

src/modules/solver/moduleSolver.f90

@@ -565,6 +565,7 @@ MODULE moduleSolver
       !Output average values
       IF (useAverage .AND. t == tFinal) THEN
         CALL mesh%printAverage()
+        CALL printAverageVTK(mesh) !TEMPORARY TO TEST VTK OUTPUT
 
       END IF