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/fvOptions/sources/general/codedSource/CodedSource.C

@@ -87,7 +87,7 @@ void Foam::fv::CodedSource<Type>::prepare
 template<class Type>
 Foam::dlLibraryTable& Foam::fv::CodedSource<Type>::libs() const
 {
-    return const_cast<Time&>(mesh_.time()).libs();
+    return mesh_.time().libs();
 }