Gets the double value stored inside a node.
If the node holds an integer value, it is returned as a double.
If the node holds a `FALSE` boolean value, `0.0` is returned; otherwise a non-zero double is returned.
If the node holds a `JSON_NODE_NULL` value or a value of another non-double type, `0.0` is returned.
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a node holding a floating point value |
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a double value. |