Update a train schedule exception

Example request

{
  "change_groups": {
    "path_and_schedule": {
      "margins": {
        "values": [
          "5%",
          "2min/100km"
        ]
      },
      "schedule": [
        {
          "arrival": "PT2H",
          "reference_base_arrival": "PT2H",
          "stop_for": "PT2H"
        }
      ]
    }
  }
}