Making hub and spoke dict callback a function of the model module instead of a separate module

[tvalenciaz]

I think having the callback that does hub and spoke dict manipulation in generic_cylinders.py as a function within the model module is more convenient because that manipulation can be dependent on the value of the config options cfg. The same could be achieved by just also passing cfg to the callback the way it is written now but in the case where the user defines a class within their model module, having the callback be a method of the class makes cfg-dependent implementation straightforward.

[bknueven]

Happy to merge if we agree on passing the config object into the callback.