- make dual time integrator tree available for insertion

Right now Zodiac can't insert any fields not required by the RHS expressions, say pressure, for output during simulations. The integrator's tree is made and fields are registered at once in the prepare_for_integration method. We need access to a valid tree for insertion before this method is called. This commit makes the tree available upon construction of the integrator. It makes no impact on integrator's API and we have tested that it works successfully in Zodiac, enabling the insertion of expressions unnecessary for the RHS.