elements keyword in the descriptor initialization

[Alireza Khorshidi]

We typically need a Gs keyword for initializing descriptor (which of course is auto-generated if not given). Gs typically has elements as keys.

Taking into account this, do we need an extra elements keyword?

https://bitbucket.org/andrewpeterson/amp/src/d292db333b4990c83a0da6d162bb0c1f27b3fd3f/descriptor/example.py?at=master&fileviewer=file-view-default#example.py-27

[andrew_peterson]

I suspect that we do still need it. This is because the model needs a standard place to look in the descriptor to see what kinds of elements are present. Not all descriptors are required to have something called Gs.