Dualtime quality-of-life improvements

• Add methods to register root expressions on a dual time integrator's execution tree, methods to lock fields (all or a particular field), and a method to copy an initialized field from STATE_N to STATE_NONE. These are tasks commonly performed by diving on pieces (tree, factory) of the integrator and it's easy to mess up. These interfaces should make it more expressive and easier to do correctly. Closes issues #47 (closed), #48 (closed), and #53 (closed).