externalize adios2py and the xarray adios2 reader

[germasch]

The idea behind this is good, since adios2py and xarray-adios2 are independently useful. The execution is horrible, though,
in that this is a unreviewable mega PR.

Basic testing succeeds, though, and whatever rough edges there are, they shouldn't be too hard to fix up.

In existing code, it'll be necessary to replace direct reading of a psc adios2 file with something like

def _open_dataset(filename: os.PathLike[Any]) -> xr.Dataset:
    ds = xr.open_dataset(filename)
    return pscpy.decode_psc(
        ds,
        species_names=["e", "i"],
        length=[1, 12.8, 51.2],
        corner=[0, -6.4, -25.6],
    )

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/pscpy/__init__.py	100.00% <100.00%> (ø)
src/pscpy/psc.py	98.03% <100.00%> (+15.11%)	⬆️