Allow multiple addons to register for the same processing step (system event)
Currently only one addon is able to register for a given processing step (i.e., system event: https://atlas-sys.atlassian.net/wiki/spaces/ILLiadAddons/pages/3149423/ILLiad+Addon+System+Events ), thus only one can be active at a time and still work successfully. It would be helpful if multiple addons could register in succession for a single system event. To give one example (but there would be many more cases with diverse addons), a library uses the ILLiad Alma NCIP addon when processing received & returned loans in Borrowing, and also wants to use the Flag Popper addon to flag certain requests for staff inspection at point of return (currently only one of these addons can work at a time).
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