Deprecate `xgcm_utils`

[jbusecke]

Triggered by #139 I reviewed the xgcm_utils module and I ultimately believe this should be refactored/removed from here.

For a bit of context, I have been using xarrayutils as a 'catchall' for things I needed to implement in certain science contexts, but a lot of it is either outdated, or should be refactored into upstream repositories.

I am going to leave this issue open for a bit to get some feedback from folks here if workflows critically depend on the xgcm_utils module.

[jbusecke]

Related to #84

[jbusecke]

Also cc'ing @TomNicholas here. Tom, there might be some bits of logic that might be helpful for xgcm.

In particular there is some logic to infer if a given input is organized on a B or C grid, which might be quite useful for https://github.com/xgcm/xgcm/issues/187