[SCHEMA] type property missing for 'height_reference'

[stephenholleran]

A data type is missing for the 'height_reference' enum.

    "height_reference": {
      "title": "Height Reference",
      "description": "The height reference frame that is used to measure the item height. E.g. onshore this is ground level i.e. the item is 0.5 m above ground level. Offshore is a bit different as it can be 20 m above mean sea level or 20 m above lowest astronomical tide.",
      "enum": [
        "ground_level",
        "mean_sea_level",
        "sea_level",
        "lowest_astronomical_tide",
        "sea_floor",
        "other"
      ],
      "default": "ground_level"
    }

Should include a data type the same as every other property in the data model has:

"type": [
        "string",
        "null"
      ],