potentialFoam: Solve for velocity potential named Phi rather than using the pressure field for this purpose

The Phi field is read if available otherwise created automatically with
boundary conditions obtained automatically from the pressure field if
available (with optional name) otherwise inferred from the velocity
field.  Phi Laplacian scheme and solver specification are required.  See
tutorials for examples.

tutorials/basic/potentialFoam/pitzDaily/system/fvSolution

@@ -17,7 +17,7 @@ FoamFile
 
 solvers
 {
-    p
+    Phi
     {
         solver          PCG;
         preconditioner  DIC;