Add support for geographical coordinates

[MuellerSeb]

lat-lon support

This PR introduces the long-awaited support for geographic coordinates given by latitude an longitude.

CovModel

• added boolean init parameter latlon to indicate a geographic model. When given, spatial dimension is fixed to dim=3, anis and angles will be ignored, since anisotropy is not well-defined on a sphere.
• add property field_dim to indicate the dimension of the resulting field. Will be 2 if latlon=True
• added yadrenko variogram, covariance and correlation method, since the geographic models are derived from standard models in 3D by plugging in the chordal distance of two points on a sphere derived from there great-circle distance zeta:
  • vario_yadrenko: given by variogram(2 * np.sin(zeta / 2))
  • cov_yadrenko: given by covariance(2 * np.sin(zeta / 2))
  • cor_yadrenko: given by correlation(2 * np.sin(zeta / 2))
• added plotting routines for yadrenko methods described above
• the isometrize and anisometrize methods will convert latlon tuples (given in degree) to points on the unit-sphere in 3D and vice versa
• representation of geographical models don't display the dim, anis and angles parameters, but latlon=True
• fit_variogram will expect an estimated variogram with great-circle distances given in radians

Variogram estimation

• latlon switch implemented in estimate_vario routine
• will return a variogram estimated by the great-circle distance (haversine formula) given in radians

Field

• added plotting routines for latlon fields
• no vector fields possible on latlon fields
• corretly handle pos tuple for latlon fields
• now only the "unstructured" keyword is recognized, everything else is interpreted as "structured" (e.g. "rectilinear")

Misc

• added EARTH_RADIUS as constant providing earths radius in km (can be used to rescale models)
• add routines latlon2pos and pos2latlon to convert lat-lon coordinates to points on unit-sphere and vice versa

Examples

• added tutorial for geographic coordinates

TODO

• kriging tutorial
• testing

[MuellerSeb]

@LSchueler this should be ready for a first review! 🥳

[MuellerSeb]

Strangely all the with self.assertRaises(ValueError) blocks in the tests don't show up on coveralls... That's why the coverage decreased.

[LSchueler]

Wow, that is quite some addition, great!

I think that we are almost good to go. Just some minor changes I hinted at in the code comments. I also think that the unittests are enough.
I'm going to push some typo fixes directly on this branch again as always ;-) So don't forget to pull before working on this again.