Fixed transient/persistent argument mixup

The classes declared in classes_def.xml were meant not to be stored, however,
the wrong XML argument name was used. The used name was 'transient' but the
code actually only looks for the name 'persistent'. This change corrects the error.
Merge pull request #1990 from Dr15Jones/fixNonPersisentClassesInPhysicsToolsFWLite

AT fix -- Fixed transient/persistent argument mixup