Expose a public interface for CF encoding/decoding functions

[shoyer]

Relevant discussion: #153

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[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=[]).