Format docstrings in Numpy style

[nanls]

comment doctrings

[pierrepo]

Please @Athenais give context. Issues are not a to do list.

We should follow the Numpy style for docstrings: http://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_numpy.html

Example:

"""
My numpydoc description of a kind
of very exhautive numpydoc format docstring.

Parameters
----------
first : array_like
    the 1st param name `first`
second :
    the 2nd param
third : {'value', 'other'}, optional
    the 3rd param, by default 'value'

Returns
-------
string
    a value in a string

Raises
------
KeyError
    when a key error
OtherError
    when an other error
"""