PropertyType Traits Class?

Is your feature request related to a problem? Please describe.
Using auto goes a long ways toward not needing to know the exact types of the inputs/results of a property type. For the times when you do need to know the type it would be nice if there was a traits class.

Describe the solution you'd like
I'd like each PT to have a corresponding traits class from which the user can get the types of the
inputs and results. Ideally this class should be auto-generated. I'm thinking something like:

// FTR the API of AOEnergy is double(const AOSpace&, const ChemicalSystem&)

// Example of getting the traits for the AOEnergy PT
using traits_type = PTTraits<AOEnergy>;

// Typedef of const AOSpace&
using input0_t = typename traits_type::input<0>;

// Typedef of const ChemicalSytem&
using input1_t = typename traits_type::input<1>;

// Typedef of double
using result0_t = typename traits_type::result<0>;

Describe alternatives you've considered
I've taken to manually writing such traits classes in some places, but I think it might be possible to auto generate one. IIRC the way the user specifies the inputs/results causes something like this to be generated under the hood already (that's how we know what types to unwrap/wrap for things like unwrap_inputs) so this issue is really just for exposing that in a user-friendly way.

Start by tracing how property types currently record input and result types for operations such as unwrap_inputs. Review the existing property-type definitions and metadata path before deciding how a public PTTraits API could expose those types. Done means users can obtain each input and result type through the proposed traits interface without manually writing traits classes.