Rewrite DataHelper.GetUpdatedDataValues to use field accessors that support indexing into repeating groups

[ivarne]

Description of the bug

If you define something like this for presentationFields in applicationmetadata, the current reflection code in DataHelper doesn't understand the indexing into a list of eiendom.

"presentationFields": [
  {
    "id": "Presentasjonsfelt-Adresse",
    "path": "eiendomByggested.eiendom[0].adresse.adresselinje1",
    "dataTypeId": "DS"
  }
],

Should be a fairly simple fix.