custom criteria keys should be listed in `keys()`

[kthyng]

I think it would be useful if for a defined custom criteria vocabulary (which can have many items defined), those that have found matches for a given Dataset have keys listed in ds.cf.keys() so that it can be checked to see if a specific custom key is present in a Dataset.

What do others think?

[dcherian]

Yes, similarly #375

[kthyng]

Ah, indeed! I'll see if I can get to it.