ENH: use readOption to fine-tune dictionary reading

- previously had 'mandatory' (bool) for advanced control of reading
  dictionary entries but its meaning was unclear in the calling code
  without extra code comments.

  Now use IOobjectOption::readOption instead, which allows further
  options (ie, NO_READ) and is more transparent as to its purpose in
  the code than a true/false bool flag was.

  This is a minor breaking change (infrequent, advanced usage only)

- minor code cleanup in dictionary lookup methods

src/functionObjects/field/fieldValues/surfaceFieldValue/surfaceFieldValue.C

@@ -1038,13 +1038,17 @@ bool Foam::functionObjects::fieldValues::surfaceFieldValue::read
             }
         }
 
-        // Mandatory if we didn't pick up a value from selectionNames_
+        // The "name" entry
+        // - mandatory if we didn't pick up a value from selectionNames_
         dict.readEntry
         (
             "name",
             regionName_,
             keyType::LITERAL,
-            regionName_.empty()
+            (
+                regionName_.empty()
+              ? IOobjectOption::MUST_READ : IOobjectOption::READ_IF_PRESENT
+            )
         );
 
         // Ensure there is always content for selectionNames_