code: function (newDatasetMetadataNames,
                            currentDatasetMetadataNames, heatMap, callback) {
      var tool = {};
      tool.execute = function (options) {
        return options.input;
      };
      tool.toString = function () {
        return 'Select Fields';
      };
      tool.gui = function () {
        var items = [
          {
            name: 'current_dataset_annotation_name',
            options: currentDatasetMetadataNames,
            type: 'select',
            value: 'id',
            required: true
          }];
        items.push({
          name: 'new_dataset_annotation_name',
          type: 'select',
          value: 'id',
          options: newDatasetMetadataNames,
          required: true
        });
        return items;
      };
      morpheus.HeatMap.showTool(tool, heatMap, callback);
    }
docs: prompt for metadata field name in dataset and in file