Rename and split `set_precomputed_quantities!`

Now that ClimaTimeSteppers has `post_explicit!` and `post_implicit!` hooks, we should probably rename `set_precomputed_quantities!` to something that better matches the hooks. In addition, `post_implicit!` only needs to compute terms needed in explicit kernels, and not in implicit kernels, so we can likely eliminate unnecessary computations there.