json schema

[KevinVeer]

LoKI-web already has a json schema available.
One should also be available for LoKI-B.

What needs to change compared to the LoKI-web version (AFAICT/understand):

• ReducedField should also allow an array with all values.
• There are many required entries which should be optional, see patch patch_req_json.txt
• StateProperty/states in the following snippet are not required. While the code seems to assume that it is. See issue runtime error triggers JSON error instead #211

         "stateProperties": {
          "type": "object",
          "properties": {
<>
            "population": {
              "$ref": "#/definitions/StateProperty"
            },
<>
          },
          "required": [
            "population"
          ]
        }