Make xgcm a dependency and remove vendored code

[VeckoTheGecko]

#1966 (comment) , and my conversations with @jbusecke highlight that we can make xgcm a normal dependency without having to vendor code.

Currently we do access xgcm.Grid._ds within our code - so that is the only remaining concern I have. We need access to the grid values as well as the staggered grid information.

We can store a copy of the ds outside of the Xgcm grid class so that we aren't relying on 3rd party private API - I think that's the best way forward.

This is a low priority change - and other v4 development will be prioritised for now