REFACTOR/DOCS: sort datasets fetch functions alphabetically

[paulinek13]

While working with the datasets, I had a few ideas:

(1) sorting the functions that fetch the datasets alphabetically in fetch_example_datasets.py (could make it easier to navigate as the file grows)
(2) also sorting them in api.rst (related #653)
(3) moving tests for those fetch functions to tests/unit/datasets

I'd love to hear your thoughts on whether these 3 changes would be useful/welcomed or if they're not really that important/needed :)

[romanlutz]

I love it, @paulinek13 !

How do you feel about splitting the file up? As you said, it is getting huge...

I agree on everything.

[paulinek13]

Yeah, I think we could split the file up and make it work similarly to how we handle the converters, so it'll be one file per fetch function

[romanlutz]

Sounds preferable to me. I personally search by file name and if there's a huge file that means I have to do two searches, so this helps me 😆 But not sure if everyone is like me.

[romanlutz]

Do you want to pick this up yourself @paulinek13 or leave it for someone else?

[paulinek13]

Yes, I'd like to pick this up