fluxRequired: Added setFluxRequired function to fvSchemes class

Added calls to setFluxRequired for p in all incompressible solvers which
avoids the need to add fluxRequired entries in fvSchemes dictionary.

Will add calls to setFluxRequired to the rest of the solvers.

tutorials/incompressible/adjointShapeOptimizationFoam/pitzDaily/system/fvSchemes

@@ -53,11 +53,5 @@ snGradSchemes
     default         corrected;
 }
 
-fluxRequired
-{
-    default         no;
-    p;
-    pa;
-}
 
 // ************************************************************************* //