Consider switching metadata encodings to JSON instead of XML

[kyleredilla]

This seems like a better option for encoding metadata. This would require corresponding API changes.

e.g., instead of

  "metadata": {
          "type": "xml",
          "global": {
            "Title": "'CMIP5 GIPL Model Output 4 km magt alt'",...

Use

  "metadata": {
          "type": "json",
          "global": {
            "Title": "'CMIP5 GIPL Model Output 4 km magt alt'",

[charparr]

I think this change would make it easier to parse this information (and more, XREF #24) in the API and reduce the number of instances where we have to manually prescribe metadata.