There are three cases that need to be handled:
 1. No coordinate
 2. Scalar coordinate (so no associated dimension)
 3. Vector coordinate (with a single associated dimension)

Currently I use `dims` as:
 1. `None`
 2. `()`
 3. `(n,)`

I'm not clear what you're proposing, perhaps using `dim` as:
 1. `None`
 2. `()`
 3. `n`

But I like that even less. Could you clarify?