SSH(x) to SSH(x) dumps x locally

In some testing today I found that SSH(JSON) to SSH(JSON) ends up dumping the JSON file in question in the local path. It turns out that odo uses the `append_sshX_to_X` function here under the `(JSON, SSH(JSON))` append signature, because the destination SSH(JSON) target is a subclass of JSON.

A quick fix to this could be just setting a rule that SSH(x) to SSH(x) raises `NotImplementedError`. The better solution long-term is probably something @mrocklin has suggested which is to refactor odo to separate the notions of data location and data format.