Allow Inheriting code_pre From Earlier Boxes #46

I received a few requests for allowing pythoncode boxes to use code from boxes before them as their code_pre, rather than specifying code directly (which often contains solutions to earlier boxes).

This is pretty doable, but we'll need to think about UI, error reporting, etc.

I received a few requests for allowing `pythoncode` boxes to use code from boxes before them as their `code_pre`, rather than specifying code directly (which often contains solutions to earlier boxes). This is pretty doable, but we'll need to think about UI, error reporting, etc.

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Allow Inheriting code_pre From Earlier Boxes #46