Wells add_completion: Add functionality to add using a completion object

Modify the add_completion method on `NexusWells` to also accept a `NexusWell` object instead of a dictionary. This will allow users to create an object instead, giving them the benefits of code completion for attributes etc.

Start at the add_completion method on NexusWells and inspect the NexusWell object interface. Confirm how the method currently handles dictionaries, then support a NexusWell input while preserving existing dictionary behavior. Done means callers can add a completion using either input form and both paths behave correctly.

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