[MISC]: Ensure we are fully compatible with expectations of xgcm usage

[jthielen]

What is your issue?

94612a3 identified a DeprecationWarning from xgcm. While it may or may not be an issue in practice, it would be nice to have a set of tests with xgcm that raise no errors or warnings with its current versions (so as to avoid any possible issues down the road). While doing this, it might also be worth verifying the current state of xwrf's approach to staggered coordinates still lines up with how xgcm does things in the more general sense (I unfortunately have not kept up myself with recent xgcm development).