Automatic Binning

[MuellerSeb]

Closes: #55 #36

This PR adds a simple automatic binning routine standard_bins to the variogram submodule.
It returns an equal division from 0 to max_dist with bin_no as number of bins. The following rules are applied:

• if max_dist is not given, it is set to one third of the box diameter
• if bin_no is not given, it is determined by sturges' rule from the number of point combinations

You can now call gs.vario_estimate without passing bin_edges. In this case, the bin_edges will be determined as described above.

[MuellerSeb]

@LSchueler ready for review. 😊

[LSchueler]

This is a really nice convenience addition!

What do you think of moving the _chordal_to_great_circle method to a more visible place and making it public?

Appart from that, I think we are ready to merge this PR.

[LSchueler]

I fixes some typos, so don't forget to pull.

[MuellerSeb]

Strange. It was merged, but this PR says it has conflicts and can't be merged. It felt like an internal GitHub Error when I clicked on "merge" and I had to reload the page. Hope nothing was broken.

Edit: after closing, it says that it was merged successfully.