Import ADIOS2 only when actually used to remove warning

[mikekryjak]

Every single time I use xBOUT I get this warning:

/home/mike/spack/var/spack/environments/hermes3/.spack-env/view/lib/python3.12/site-packages/xarray/backends/plugins.py:109: RuntimeWarning: Engine 'bout_adios2' loading failed:
No module named 'adios2'
  external_backend_entrypoints = backends_dict_from_pkg(entrypoints_unique)

This PR fixes this by making sure that from adios2 import FileReader is done only when xarray actually needs it, i.e. in the ADIOS2 backend.

Unfortunately as I have never used ADIOS2 and it is completely untested as far as I can tell, I cannot verify that this doesn't break loading with it... although it is a simple change.

Perhaps @pnorbert or someone else with ADIOS2 experience could review.

[mikekryjak]

David made a better one: #343 (review)

[dschwoerer]

@pnorbert if you think this is wrong, please open a bug, or a PR with a different solution.