NeuralNetwork (de)serialization does not preserve activation function

Currently the (de)serialization features of the NeuralNetwork library does not preserve the activation function. The default activation function (currently sigmoid) will be used after deserialization.

This could be fixed by adding a `replacer` function to [`JSON.stringify()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify).

Start at the NeuralNetwork serialization and deserialization implementation, then inspect the JSON.stringify call mentioned in the issue. Done means a serialized network restores its original activation function rather than defaulting to sigmoid; verify the relevant serialization behavior with the project’s existing checks.

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