message: add minimal documentation
diff: @@ -85,8 +85,15 @@ 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.
  - TODO
  - 1/(r/r0)^m
  + It requires the argument "func" to be one of the following:
  + 1) "poly-m"
  + 2) "poly-3m"
  + 3) "exp"
  +
  + 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
  + to infer it.
  +
  rbf (str): The radial basis functions to use. The available options are:
  