Add devices names to link.__str__ for "self links"

[Andrew-Dickinson]

This may be a bit controversial since it makes things a bit bulky at times, but looking at the string representations for "self links" in slack is very confusing.

NN7489 -> NN7489 is so much more confusing than nycmesh-7489-epr7 -> nycmesh-7489-north

For now, we only include the device names for self-links (links between devices within a single node)

This heterogenous format does look a bit messy at times, but results in a lot more clarity for single links in isolation

Also switches to a double headed arrow to indicate that links are not directional

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.11%. Comparing base (df26131) to head (7c97877). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #703 +/- ## ========================================== + Coverage 94.06% 94.11% +0.04% ========================================== Files 82 82 Lines 3406 3415 +9 ========================================== + Hits 3204 3214 +10 + Misses 202 201 -1 ```

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[WillNilges]

I would be OK with changing it to the device name everywhere, since all devices ought to have the NN in the name anyway.

[Andrew-Dickinson]

You're sure that isn't too verbose? They're so long....

[Andrew-Dickinson]

Let's keep it this way for now, and if we decide we don't like the inconsistency later we can tweak it