Bring closures closer to best practices

[nicholaskl97]

Is your feature request related to a problem? Please describe.

Lots of functions are passed around in this package, some of which capture variables in a way advised against by the Julia performance tips. Those advise use of let blocks for better performance and type stability. That's probably among the many things that could be done to speed up performance of this library.

Describe the solution you’d like

Check wherever closures are made in this library, such as the NumericalNeuralLyapunovFunctions function, and bring them up to best practices, likely with let blocks.