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/primitives/functions/Function1/Function1/Function1New.C

@@ -54,12 +54,16 @@ Foam::Function1<Type>::New
             << "For " << entryName << " with dictionary entries: "
             << flatOutput(coeffs->toc()) << nl;
 
+        // The "type" entry - mandatory if no redirectType provided
         coeffs->readEntry
         (
             "type",
             modelType,
             keyType::LITERAL,
-            modelType.empty()  // "type" entry is mandatory if no 'redirect'
+            (
+                modelType.empty()
+              ? IOobjectOption::MUST_READ : IOobjectOption::READ_IF_PRESENT
+            )
         );
 
         // Fallthrough