Validate Input/Output class for new decorator functions at build time

[elliotgunton]

Is your feature request related to a problem? Please describe.
When writing a script template I might forget to use an Input or Output in the function definition, when subclasses of those are the only things allowed.

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Validate and raise an error when function input/output types are not Input/Output (allow empty/None input/output)

Describe alternatives you've considered
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Remember to use the right types? 😆

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