Test identifier functions

[daxkellie]

The group of functions random_id(), sequential_id() and composite_id() need to be tested for robustness. These functions can auto-build unique identifiers for users, and importantly, they shouldn't be allowed to be overwritten once they exist in a dataset (otherwise, they are no longer useful identifiers).

It might be worth checking whether they work as expected, and ensure that the messages returned in the console are nice and clear about what they are doing. It also would be good to return errors when they are used within columns or functions they aren't meant for (e.g., use_coordinates() and the supported spatial columns).