public Double getDouble(String featureName) {
        Tensor value = getTensor(featureName);
        return value == null ? null : value.asDouble();
    }
Returns the value of a scalar feature, or null if it is not present.

@throws IllegalArgumentException if the value exists but isn't a scalar
                                 (that is, if it is a tensor with nonzero rank)
container-search/src/main/java/com/yahoo/search/result/FeatureData.java