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/functionObjects/functionObjectList/functionObjectList.C

@@ -771,7 +771,7 @@ bool Foam::functionObjectList::read()
 
         addProfiling(fo, "functionObjects::read");
 
-        const_cast<Time&>(time_).libs().open
+        time_.libs().open
         (
             functionsDict,
             "libs",