Time integrator interface updates

[gardner48]

Summary

Remove const on input state to time integrator right-hand side (RHS) functions to allow filling ghost cells within the RHS call. Remove the set_pre_rhs_action and set_post_update functions in the time integrator interface. Update documentation to reflect changes from #3984.

Additional background

Before the updates in #3984 a set_post_update was used attach a function for filling ghost cells after computing a new stage or solution in a time integrator. #3984 expanded the time integrator interface with SUNDIALS to support implicit and ImEx methods which require updating the ghost cells for RHS evaluations inside iterative solvers. To support these methods, a new set_pre_rhs_action function was added to attach a function for filling ghost cells. This created some confusion/inconsistencies in how to fill ghost cells between explicit and implicit methods. To simplify things, this PR removes const from the input state to a RHS function so its ghost cells can be filled as part of the RHS callback and removes the pre_rhs_action/set_post_update functions.

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

cc: @ajnonaka

[ajnonaka]

The amrex-tutorial SUNDIALS example checks out fine. Thanks for the updates and the documentation!