message: add proper api docs for soap weighting
diff: @@ -85,10 +85,25 @@ class SOAP(Descriptor):
  sigma (float): The standard deviation of the gaussians used to expand the
  atomic density.
  weighting (dict): The parameters of the radial weighting function.
  - It requires the argument "func" to be one of the following:
  - 1) "poly-m"
  - 2) "poly-3m"
  - 3) "exp"
  + It requires the argument "func" to be one of the following
  +
  + :"poly-m": :math:`w = \\left\{\\begin{matrix} \\frac{1}{(\\frac{r}{r_0})^{m}} & c = 0 \\ \\frac{1 + c}{c + (\\frac{r}{r_0})^{m}} & c > 0 \\end{matrix}\\right.`
  +
  + For reference see:
  + "Willatt, M., Musil, F., & Ceriotti, M. (2018).
  + Feature optimization for atomistic machine learning yields a data-driven
  + construction of the periodic table of the elements.
  + Phys. Chem. Chem. Phys., 20, 29661-29668.
  + "
  +
  + :"poly-3m": :math:`w = (1 + 2 (\\frac{r}{r_0})^{3} -3 (\\frac{r}{r_0})^{2}))^{m}`
  +
  + For reference see:
  + "Caro, M. (2019). Optimizing many-body atomic descriptors for enhanced
  + computational performance of machine learning based interatomic potentials.
  + Phys. Rev. B, 100, 024112."
  +
  + :"exp": :math:`w = exp^{-m r}`
  Apart from the decay-function-specific parameters, a "threshold" can be
  specified (default: 1e-3). The threshold is only used if rcut=None in order
  