Add ``swrm`` mode argument in the executable.

[YooSunYoung]

We want to allow users(scientists) to reduce the file while being written.
In that case we need to allow swrm mode.
I would like this option to be explicitly chosen by users.

[jl-wynen]

Can you explain why you want users to choose? Esp. considering scipp/essreduce#181

[YooSunYoung]

Thanks! I forgot about that discussion.
Because we want to be explicit about the reduced files, if they are from intermediate raw files or final raw files.

If we use swrm mode by default, we should just consider different approach...
Like... manually reduced files are always considered as temporary files...