Rename equivariant_model

[cortner]

It doesn't actually produce a model but a basis. So let's call it that.

equivariant_basis

But before we rush into it, let's think this through. This is the canonical construction of an equivariant ACE basis. There will be others ... so let's specify that too. So maybe the right choice is

equivariant_ace_basis

@CheukHinHoJerry @zhanglw0521 -- thoughts?

[zhanglw0521]

Jerry suggested equivariant_ace_descriptor which might make more sense?

[cortner]

But I'm ok with that too, but depends who we are talking to. I'm not convinced we should so readily give in to the ML language.

I'd be even open to just having equivariant_ace_descriptor to be the alias for equivariant_ace_basis (or vice-versa).

One more thought before you make the change: should the equivariant even be there? It's really implicit, isn't it? But I'm also ok to keep it for now ...