change combine_responses derivation?

[gergness]

While debugging a feature in the webapp, discovered that combine_responses works in more places when you give it subvariable urls rather than category_id as rcrunch currently does. I suspect that this is because rcrunch still uses an old API that works in some situations, and it would be beneficial to change to the new API, but I may be misunderstanding something.

Example of combine_responses working inside a make_array call which only works when using variables.

(The other issue it might fix is allowing expressions in the args)

{
    "derivation": {
        "function": "array",
        "args": [
            {
                "function": "make_frame",
                "args": [
                    {
                        "map": {
                            "1": {
                                "function": "combine_responses",
                                "args": [
                                    {
                                        "variable": "https://alpha.crunch.io/api/datasets/db8ed626d77a43669799d1b28eb53d12/variables/2281b47ff65f4b41896d06e2b2220fbe/subvariables/00001/"
                                    },
                                    {
                                        "variable": "https://alpha.crunch.io/api/datasets/db8ed626d77a43669799d1b28eb53d12/variables/2281b47ff65f4b41896d06e2b2220fbe/subvariables/00000/"
                                    }
                                ],
                                "references": {
                                    "name": "Channel 4 + BBC"
                                }
                            },
                            "2": {
                                "variable": "https://alpha.crunch.io/api/datasets/db8ed626d77a43669799d1b28eb53d12/variables/2281b47ff65f4b41896d06e2b2220fbe/subvariables/00000/"
                            },
                            "3": {
                                "variable": "https://alpha.crunch.io/api/datasets/db8ed626d77a43669799d1b28eb53d12/variables/2281b47ff65f4b41896d06e2b2220fbe/subvariables/00001/"
                            },
                            "4": {
                                "variable": "https://alpha.crunch.io/api/datasets/db8ed626d77a43669799d1b28eb53d12/variables/2281b47ff65f4b41896d06e2b2220fbe/subvariables/00002/"
                            }
                        }
                    },
                    {
                        "value": [
                            "1",
                            "2",
                            "3",
                            "4"
                        ]
                    }
                ]
            }
        ]
    },
    "name": "R combine_response in a make_frame (flipped order)",
    "alias": "r_combine_response_in_make_frame_flipped"
}