Evaluate whether raw data can be sent with the REST API schemas

[blootsvoets]

The REST API now uses generic names like Dataset and header but assumes that all data will be aggregated and from a sensor. To support raw data and questionnaires as well, we should look which fields could be nullable or generalised.

As per @fnobilia:

To use this header with raw data, I would suggest to make descriptiveStatistic and timeFrame nullable. But to return raw data using Dataset, the Item schema has to support all raw data schemas and startDateTime can be nullable. Just to make it clearer

{
  "namespace": "org.radarcns.avro.restapi.dataset",
  "type": "record",
  "name": "Item",
  "doc": "Dataset item. It contains a value and its related time.",
  "fields": [
    {"name": "sample", "type": [
      "org.radarcns.avro.restapi.data.DoubleSample" ,
      "org.radarcns.avro.restapi.data.Acceleration",
      "org.radarcns.empatica.EmpaticaE4Acceleration",
      "org.radarcns.biovotion.BiovotionVSMAcceleration",
      "..."
      ], "doc": "..." } ,
    {"name": "startDateTime", "type": ["null", "string"], "doc": "...", "default": null}
  ]
}

[...] This change implies some changes also in the REST-Api project to return JSON

This would also affect the header org.radarcns.avro.restapi.header.Header.