Consider using SWMR by default

[jl-wynen]

          Are we using `swmr=True` when opening the `hdf5` files in the nightly tests? We should, it will help making the file access more robust.

Originally posted by @jokasimr in #178 (comment)

[jokasimr]

I meant in the integration tests in Gitlab, because there we are reading from the ess file storage.

[jl-wynen]

I missed that part of your comment. In principle we can. But that would mean that the tests don't represent the production environment.