Arrays as attributes

Created by: matthew-d-jones

Attributes must be allowed to be arrays, for example for the vector attributes of an NXtransformation:

{
  "values": 12.064,
  "type": "dataset",
  "name": "location",
  "dataset": {
    "type": "double"
  },
  "attributes": {
    "transformation_type": "translation",
    "units": "m",
    "NX_class": "NXtransformation",
    "depends_on": ".",
    "vector": [
      0.0,
      0.0,
      -1.0
    ]
  }
}

Blocked by #56 (closed)