faceSource: Writing the total area of the faceSource (sum(magSf)) for each time is now optional

Previous behavior which may be useful for moving-mesh cases can be
selected using the optional entry:
    writeTotalArea  yes;

The initial total area is written in the log and data file header e.g.:

 #   Source : faceZone f0
 #   Faces  : 8
 #   Area   : 1.063860e-02

tutorials/multiphase/potentialFreeSurfaceFoam/oscillatingBox/system/controlDict

@@ -75,6 +75,7 @@ functions
         outputControl   timeStep;
         outputInterval  1;
         log             yes;
+        writeTotalArea  no;
         valueOutput     no;
         source          faceZone;
         sourceName      f0;