Add alias .to_file() for .to() method of structures and molecules

[QuantumChemist]

Summary

I have noticed that many beginners (including myself a while ago) find it very unintuitive to write e.g. structures to a file by using .to(), especially with the .from_file() method counterpart. That's why I added aliases to_file for structures and molecules.

PS: This is the second attempt of #3354 as something went wrong there...

[shyuep]

You don't actually need to write it this way. All that needs to be done is to add to_file = to and write_file = to in SiteCollection. That will create the aliases without so much code.

[QuantumChemist]

You don't actually need to write it this way. All that needs to be done is to add to_file = to and write_file = to in SiteCollection. That will create the aliases without so much code.

I know, that's how I started, but when I tried to call the alias in pycharm, it wouldn't give me the input hints and at least for me they are really important. If you think that's not necessary, then I can change it to the simple alias.

Also when I only added to_file = to in SiteCollection, it would throw an error saying something like "cannot call abstract method" (I'm writing this from my phone, I would need to check the details tomorrow again if you wanna know the exact error)

[janosh]

We definitely need a test for the new alias.

[QuantumChemist]

We definitely need a test for the new alias.

Aye aye! 🫡

[QuantumChemist]

hi @janosh I hope the tests are ok like that?

[janosh]

Thanks @QuantumChemist! 👍