update incentive_relationship_schema

[matelau]

To make exporting the incentive relationship easier I have added the explicit type of prereq incentive relationship to each property in the prerequisite object. Transforming

  "prerequisites": {
    "CO-541": "CO-455",
    "CO-542": "CO-457"
  }

to

  "prerequisites": {
    "CO-541": {
      "allof": [
        "CO-455"
      ]
    },
    "CO-542": {
      "allof": [
        "CO-457"
      ]
    }
  },

For more examples see: #531

This change should be backwards compatible with the existing spreadsheet-to-json scripts, but requires relaxing of the incentives relationship schema from oneOf to anyOf.