Refactor Output Keys

[JackGeraghty]

Each node currently requires an output_key, this key can be set to None but in the instance where it isn't it holds a string value. This forces a node to have at most one output_key. From implementing some of the ViSQOL nodes there is a need for multiple values to be added to the results dictionary within a single node.

A potential refactor of this functionality would include changing the output key to be either a string OR a list of strings. When implementing a node an order can then be defined for the output keys and consequently ordered as such in a JSON definition..

Could look like for example:

{
    "node": {
        "output_keys": ["key_one", "key_two", ...]
}