Xarray bridge wip

[pp-mo]

Proof-of-concept code.

N.B. test failures probably need updating with iris/main + lockfiles
Beyond that, who knows?

Currently planning to host this in a new repo "scitools/ncdata"
(probably)

Now updated with netcdf interface.

Lots to do here:

• throw at much wider range of data + resolve additional problems
• tests + more tests
• support groups (a bit)
• support unlimited dimensions
• support Xarray 'encoding' for save controls ?
• support Iris + Xarray load/save options within conversion operations
• install in own repo, organise all the usual including GHA, RTD etc
  • docstrings + API docs build
• update to iris/main latest (where required, once separate?)
• thought will be needed to get working with netCDF > 1.6.0
  • not clear how thread-safety can be resolved when building lazy arrays for both Iris and Xarray to use ???