nan instead of NaN in covjson responses (from hyrax-docker)

[ndp-opendap]

User @kazz-s reported this issue in hyrax-docker, but the truth is it belongs here.

@kazz-s said:

I have a case where some application/prs.coverage+json responses, look like this:

{
  ...
  "ranges": {
    "orog": {
      "type": "NdArray",
      "dataType": "float",
      "axisNames": ["y", "x"],
      "shape": [1, 1],
      "values": [nan]
    }
  }
}

numpy somehow handles this but most json parsers expects NaN constant not nan, can this be fixed somehow?

The problem is in the modules/fileout_covjson code and must be addressed there.