Expose a public interface for CF encoding/decoding functions

[shoyer]

Relevant discussion: #153

[stale]

In order to maintain a list of currently relevant issues, we mark issues as stale after a period of inactivity

If this issue remains relevant, please comment here or remove the stale label; otherwise it will be marked as closed automatically

[jhamman]

@shoyer - I know we've made some steps towards addressing some of this issue (e.g. #1752, #1803). What do you think it would take to make that the coding module public API?

[shoyer]

I think the eventual interface we want is something like this:

• CF decoding in open_dataset is controlled by a coders argument, which takes a list of coders to apply in order.
• CF encoding in to_netcdf is also controlled by a coders argument, which takes a list of coders to apply in reversed order.
• Helper functions (get_coders()?) make it easy to construct the default coders, e.g., with boolean options for disabling particular coders.

To complete this, we'll need to get all CF encoding/decoding happening in Coder classes. VariableCoder might not suffice -- we also need a way to indicate transformations of a full dataset (e.g., for coding/decoding coordinates).

Eventually, it should be possible to read a dataset without any CF decoding (e.g., by setting coders=[]).

[TomNicholas]

Can these coders just be new zarr codecs? Exposing xarray's decoding logic in a way that follows that interface would allow for zarr to become a "universal reader" - see zarr-developers/zarr-specs#303.