ENH: mutable libs() access in Time and other classes (#1737)

- This reflects the pre-existing coding situation where const_cast was
  used throughout to effect the same.

STYLE: fix private/protected access

- CodedField, codedMixedFvPatchField

src/OpenFOAM/db/dictionary/functionEntries/codeStream/codeStream.C

@@ -73,7 +73,7 @@ Foam::dlLibraryTable& Foam::functionEntries::codeStream::libs
     (
         dict.topDict()
     );
-    return const_cast<Time&>(d.time()).libs();
+    return d.time().libs();
 }