`mapping.json` needs to give complete source object

[cmutel]

Given a source object (from database-1.json):

  {
    "name": "Iodine-131",
    "context": [
      "Emissions to water",
      ""
    ],
    "unit": "kBq"
  },

The mapping produced is:

  {
    "source": {
      "name": "Iodine-131",
      "context": [
        "Emissions to water",
        ""
      ]
    },
    "target": {
      "uuid": "618c3a74-5b36-4d8a-b94f-26b8270448f8",
      "name": "Iodine-131",
      "context": "water/unspecified",
      "unit": "kBq"
    },
    "conversionFactor": 1,
    "comment": "Identical names"
  },

That's correct, but could fail in the future because it doesn't specify the source unit. We have already seen the value for this column can vary, and different units would require different conversion factors.