[Feature Request] inventory field

[Monica-Wood]

This is slightly related to issue #1636.
If needing lots of duplicates for the reasons of loans/library checkouts, it might be better to have an inventory field (or a way to set which field is used as inventory) that holds how many of an object there is. However the inventory would need to be connected somehow to the loans or library function in that it shows how many are available and how many are out on loan/checked out.

Happy to discuss pros and cons on this (on here or slack).