Getter method for available dft functionals / expose in python

[puckvg]

The available dft functionals are enumerated in functionals.json in the cpp layer and enumerated in the documentation but it is not exposed in the python layer.
It would be useful to be able to enumerate the available functions in the python layer as well.

[nabbelbabbel]

This listing in the json file is only true for the functionals provided by qdk/chemistry.
Any plugin can have differing naming and/or a differing set of methods.
I do not think that we will or can impose a strict set or a naming convention on plugins for their settings values.
For this reason, any solution to this would have to work with plugins too.

The existing method for that is to set the allowed settings values for the specific settings such that all functionals are listed.
This automatically allows for queries.

The missing piece here is to allow for derived settings to modify the defaults and allowed options of already added settings via set_defaults.

[wavefunction91]

Agreed. I think this is a nice-to-have, as we do already list this in the Sphinx documentation. This can get a bit fragile if it's not executed properly. Let's see where we are later in the week, but for now, I'm going to un-tag this for Release 1 in favor of things that must happen.