Update hyper-parameters to a more extensible schema

[Luthaf]

There are two main ingredients to a spherical expansion:

• the density being expanded
• the basis on which to make the expansion

Additionally, real space/short range (i.e. SOAP) density expansion requires the definition of local environments with a cutoff radius and smoothing functions.

This PR refactor the hyper-parameters of spherical expansion and related calculators to reflect this structure and make is easier to add new variations in the density and basis ingredients.

TBD:

• code to automatically convert from old hypers
• update examples & tutorials
• rewrite all of splining code
• should we expose classes for the hyper building blocks? I.e. allow something like

cutoff = Cutoff(
    radius=5.0,
    smoothing=ShiftedCosineSmoothing(width=0.5),
)

density = GaussianDensity(width=0.3)

basis = TensorProductBasis(
    max_angular=4,
    radial=GtoRadialBasis(max_radial=6),
)

calculator = SoapPowerSpectrum(cutoff, density, basis)

[github-actions]

Here is a pre-built version of the code in this pull request: wheels.zip, you can install it locally by unzipping wheels.zip and using pip to install the file matching your system

[jwa7]

I haven't properly reviewed the code, but I've beta-tested the functionality and to me the new structure of the hypers makes sense, and the error message for converting from old hypers is very helpful. :)