Rename `region_observer!` to `observer!` in `alternating_update`, which is more consistent with the notation of `ITensors.dmrg`, `ITensorTDVP.tdvp`, etc.
Rename
region_observer!toobserver!inalternating_update, which is more consistent with the notation ofITensors.dmrg,ITensorTDVP.tdvp, etc.