Add `BeginStep` and `EndStep` as part of the context manager

[finsberg]

According to https://adios2.readthedocs.io/en/v2.9.2/components/components.html#beginstep we should always make sure to wrap all reads and writes into begin - end step blocks, so we should just make sure to add this as a part of the context manager so that we don't forget to do this.