Add path template "sunique" to disambiguate between singleton tracks

[jaimeMF]

Solution for #4436. It is mostly copied from the aunique template. I don't know if there could be better defaults for the disambiguators.

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

[sampsyo]

Wow! It's pretty cool that it was possible to make this work. I would have suspected it would be more complicated.

It seems like there are two major things we'd need to do to make this merge-ready:

• Abstract out the shared functionality between aunique and sunique. We'd want to avoid any code duplication by writing a single method that can work like both, and then turn the two template functions into different calls to that function.
• Make sure _memotable is handled correctly elsewhere. It's a complicated bit of functionality that requires invalidation whenever the database changes. We'll want to make sure that database changes that could affect sunique correctly trigger this invalidation.

[jaimeMF]

It seems like there are two major things we'd need to do to make this merge-ready:

* Abstract out the shared functionality between `aunique` and `sunique`. We'd want to avoid any code duplication by writing a single method that can work like both, and then turn the two template functions into different calls to that function.

I will refactor it to a common method.

* Make sure `_memotable` is handled correctly elsewhere. It's a complicated bit of functionality that requires invalidation whenever the database changes. We'll want to make sure that database changes that could affect `sunique` correctly trigger this invalidation.

I will try to have a look at how is handled and if any change is needed.

[jaimeMF]

* Abstract out the shared functionality between `aunique` and `sunique`. We'd want to avoid any code duplication by writing a single method that can work like both, and then turn the two template functions into different calls to that function.

I have moved the common functionality to a single method.

* Make sure `_memotable` is handled correctly elsewhere. It's a complicated bit of functionality that requires invalidation whenever the database changes. We'll want to make sure that database changes that could affect `sunique` correctly trigger this invalidation.

I have only found references to _memotable in the library.py file and it seems to be only used for those two templates, an since each one includes its name in the key they won't conflict. The existing invalidation should still work.

[sampsyo]

Wow; awesome work!! This is looking very snazzy already.

Would you mind trying your hand at the documentation? We'll want to add a section about sunique that ideally makes reference to the aunique docs (again avoiding the need for exhaustive duplication of the same concepts). We will also need a brief changelog entry.

[jaimeMF]

Would you mind trying your hand at the documentation? We'll want to add a section about sunique that ideally makes reference to the aunique docs (again avoiding the need for exhaustive duplication of the same concepts). We will also need a brief changelog entry.

I will do it in the following days.

[jaimeMF]

I have added a short descriptions to the docs and an entry to the changelog.

[sampsyo]

Truly awesome!! Thank you for the fast and high-quality work addressing this. I'm super impressed this was achievable without too much fuss! 👏