Add new scattering_process_struct_init function for union processes

[tkittel]

Adding new scattering_process_struct_init function and apply it in all union processes. The ulterior motive is that this will allow us to add to or modify this struct in the future without having to edit all the union processes.

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Development OS / boundary conditions

Tested on Ubuntu 24 + conda with latest McCode and mctest --instr=union

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[mads-bertelsen]

This is a nice pattern, certainly catches some annoying bugs before they happen and lets us add new control parameters with a sensible default without altering all process components. It can also be expanded with the common inputs, such as interact_fraction abd the process enumerator that is required for all processes.

Similar initialization functions should exists for all the major structs, geometry, logger and so forth.