Alias VaspInputSet to VaspInputGenerator

[janosh]

To accomodate atomate2 terminology.

Closes #3565.

@utf @davidwaroquiers Let me know if this serves your purpose and if you think more changes along similar lines would make sense.

[davidwaroquiers]

Thanks @janosh !
I would add the aliases to the subclasses too. For the base VaspInputSet, it's almost never used directly so it would not clarify things for the users. I can do that and make a PR to your vasp-input-gen-alias. I think it would be useful to add some explanations or note on this in the documentation. I will see to add this somewhere, at least in the API docstrings somewhere, maybe also somewhere in the main documentation, but I don't think there is anything (yet) about the io sets there.

[janosh]

I can do that and make a PR to your vasp-input-gen-alias. I think it would be useful to add some explanations or note on this in the documentation

please, doc improvements are always welcome!

I would add the aliases to the subclasses too.

there are 23 subclasses that inherit from VaspInputSet via DictSet. not sure we need aliases for all of them. but only adding it for some seems inconsistent. not sure what's best

[janosh]

@davidwaroquiers this PR is conflicting now so I'm leaning towards resolving and merging now. you can then make a subsequent PR straight to master