Standardised name of area-per-unit-volume to Av

This change applies to diameter models within the multiphaseEuler
module, heat transfer fvModels, and the LopesdaCosta porosity and
turbulence models.

User input changes have been made backwards-compatible, so existing
AoV/a/Sigma/... entries and fields should continue to work.

tutorials/modules/CHT/heatExchanger/constant/porous/fvModels

@@ -25,7 +25,7 @@ porousToair
 
     semiImplicit    no;
 
-    AoV             200;
+    Av              200;
 
     heatTransferCoefficientModel constant;