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/OpenFOAM/expressions/exprString/exprString.H

@@ -175,7 +175,7 @@ public:
         (
             const word& keyword,  //!< Lookup key. Uses LITERAL (not REGEX)
             const dictionary& dict,
-            bool mandatory = true
+            IOobjectOption::readOption readOpt = IOobjectOption::MUST_READ
         );
 
         //- Read/expand optional entry with dictionary variables,