added estimator madogram

[ibkvictor]

added estimator F - madogram

|f_diff|

[LSchueler]

Hi Victor,

I am truly sorry for keeping you waiting so long for a response! I like the idea of supporting the madogram. But before we can merge your PR, there are a few small things I would like to see changed.

The new functionality is not mentioned anywhere in the docstrings. Could you add some text to the docstring of the vario_estimate function in the file variogram.py? And could you also add a few simple unit tests?
Apart from those two points and the very small comments I made inline, I think we are nearly good to go. Thanks again!

Cheers,
Lennart

[LSchueler]

Thank you for the changes.
However, now there is an inconsistency between the argument to choose your estimator, which is "madogram" and the docstring describing the estimator, which is "f-madogram". Please fix that.
And another problem in the docstring is the missing references. Have a look at the last lines of the docstring

    References
    ----------
    .. [Webster2007] Webster, R. and Oliver, M. A.
           "Geostatistics for environmental scientists.",
           John Wiley & Sons. (2007)

You should include your references there too and then you can link them via [Cooley2006]_ and [Neves2011]_.