Update NDG structure in `default_params.json`?

Observation: should the default_params.json have an updated structure for non-demographic variants?

We added an upgrade step in #424. Doesn't look like default params file changed. Should it have been? New params files do come out as expected though.

Current in params.json (simple single-level structure):

  "non-demographic_adjustment": {
    "ip": {},
    "op": {},
    "aae": {}
  },

What the params.json looks like after passing though inputs (example):

  "non-demographic_adjustment": {
    "variant": "variant_2",
    "value-type": "year-on-year-growth",
    "values": {
      "aae": {
        "ambulance": [1.0121, 1.0142],
        "walk-in": [1.0121, 1.0142]
      },
      "ip": {
        "elective": [1.0053, 1.0071],
        "maternity": [1, 1],
        "non-elective": [0.9955, 1.0017]
      },
      "op": {
        "first": [1.0228, 1.027],
        "followup": [1.0228, 1.027],
        "procedure": [1.0228, 1.027]
      }
    }
  },

Start by reading the upgrade step from #424 and comparing it with default_params.json, especially the non-demographic_adjustment section. Check whether the default file should use the same structure shown in the generated params.json example; done means the expected default structure is established and the input flow remains valid.