Fix Python compatible `StateHD<D>`s and respective dual numbers about feos

[prehner]

Since we have to deal with this problem now anyways: Is there any way we can make the implementation of the Python FFI for the dual numbers more ergonomic? It's virtually impossible to make a Box<dyn DualNum<f64>> using a PyDualNumber<PyObject> and doing a (manually implemented) type check for every operation we have also seems unfeasible.

Aside from that the steps that you outline seem doable. Maybe expose ALL dual numbers that are created in the num-dual Python FFI and then in feos we can just build the States that we need. E.g., currently num-dual builds dual numbers with up to 10 derivatives but we would only need them for up to 2 in feos.

(For more than 10 derivatives backward AD should be seriously considered anyways)

[g-bauer]

These are things to consider:

1. which dual numbers do we generate Python bindings for?
2. which do we actually export in a num-dual module (the Python package)

For feos we just have to make sure that all variants we need in HelmholtzEnergy are build and public in the python (Rust) module. But I agree, we should investigate if any improvements to the way we build the bindings can be made. This is, however, an issue for num-dual.

In feos, currently two macros are used. One implements the getters for the states that are at the moment defined manually, the second one implements the HelmholtzEnergy trait. It should be possible to do that all automatically given a list of dual numbers.