xarray adios2 support

[pnorbert]

Added xarraybackend.py to xBOUT as it is bout++ specific code not generic adios solution.
Open .bp files with "bout_adios2" engine in xarray.
It requires BOUT-dev #3098

Tested with elm-pb/xpt run and the following script:

import xbout
import matplotlib.pyplot as plt

# Read the data
ds = xbout.open_boutdataset('BOUT.dmp.bp',
        geometry="toroidal",
        gridfilepath="g185472-68x64-curv.nc"
     )

# Interpolate, mapping field-aligned coordinates
ds_highres = ds.bout.interpolate_parallel("P", n = ds.metadata['zperiod'])

# Plot
ds_highres.isel(t=-1, zeta=0)["P"].bout.pcolormesh(shading="gouraud")
plt.show()

[pnorbert]

Can someone please explain what I need to fix to pass the tests?

[bendudson]

Hi @pnorbert ! Thank you very much for this! It's great to have this capability.
The linting and black failures should be just a matter of running flake8 and black on the new code.

The other failures are strange: Segfaults shouldn't happen from python code. Python 3.10 is also segfaulting in xBOUT (https://github.com/boutproject/xBOUT/).