ENH: use dictionary::readEntry for detection of input errors (#762, #1033)

- instead of   dict.lookup(name) >> val;
  can use      dict.readEntry(name, val);

  for checking of input token sizes.
  This helps catch certain types of input errors:

  {

      key1 ;                // <- Missing value
      key2 1234             // <- Missing ';' terminator
      key3 val;
  }

STYLE: readIfPresent() instead of 'if found ...' in a few more places.

src/finiteVolume/functionObjects/fieldSelection/fieldSelection.C

@@ -50,7 +50,7 @@ Foam::functionObjects::fieldSelection::~fieldSelection()
 
 bool Foam::functionObjects::fieldSelection::read(const dictionary& dict)
 {
-    dict.lookup("fields") >> *this;
+    dict.readEntry("fields", *this);
 
     return true;
 }

src/finiteVolume/functionObjects/volRegion/volRegion.C

@@ -111,7 +111,7 @@ bool Foam::functionObjects::volRegion::read
     {
         case vrtCellZone:
         {
-            dict.lookup("name") >> regionName_;
+            dict.readEntry("name", regionName_);
 
             regionID_ = mesh_.cellZones().findZoneID(regionName_);