Add "Parameters" and "Returns" to all functions

[hagenw]

Reviewing the source models in sfs/mono/source.py I realized that some of them have documentation explaining the input and output parameters using

Parameters
----------
omega : float
    Frequency of source.
x0 : (3,) array_like
    Position of source.

Returns
-------
XyzComponents
    Particle velocity at positions given by `grid`.
    See :class:`sfs.util.XyzComponents`.

Maybe it is a good idea to add this to all functions as it helps a new user a lot, while reading the API.

I can help to add this to the existing ones. This issue is more about that we should start to add this to all new functions we are going to implement.