Add sum model

[MuellerSeb]

This PR adds a SumModel class to represent sums of Covariance models.

Sum-Model

• added SumModel class
  • represents sum of covariance models
  • behaves just as a normal covariance model with kriging and field generation
  • covariance models can be added with overloaded + operator: model = m1 + m2
  • class is subscriptable to access sub-models by index: m1 = model[0]
  • included models will get a nugget of 0 and the nugget is stored separately in the sum-model
  • model variance is the sum of the sub-model variances
  • model length-scale is weighted sum of sub-model len-scales, where the weights are the ratios of the sub-models variance to the sum variance (motivated by the integral scale, which satisfies this relation)
  • anisotropy and rotation need to be the same for all included sub-models
  • parameters of the sub-models can be accessed by name with added index suffix: model[0].nu == model.nu_0
  • fitting: if len_scale is fixed, none of the len_scale_<i> can be fixed since len_scale is calculated from variance ratios
• added Nugget class (empty SumModel)
  • allow len scale of 0 in CovModel to enable a pure nugget model
  • added zero_var and model attributes to Generator ABC to shortcut field generation for pure nugget models

import gstools as gs
m1 = gs.Gaussian(dim=2, var=1.0, len_scale=2.0)
m2 = gs.Gaussian(dim=2, var=2.0, len_scale=20.0)
model = m1 + m2
model.plot()

Other changes

• removed var_raw attribute from CovModel (was rarely used and only relevant for the truncated power law models)
  • BREAKING CHANGE (but not to many should be affected)
  • TPLCovModel now has a intensity attribute which calculates what var_raw was before
• simplified variogram fitting (var_raw was a bad idea in the first place)
• variogram plotting now handles a len-scale of 0 (to properly plot nugget models)
• fitting: when sill is given and var and nugget are deselected from fitting, an error is raised if given var+nugget is not equal to sill (before, they were reset under the hood in a strange way)

[MuellerSeb]

@LSchueler this is ready for review.

[LSchueler]

Awesome, I'll need some time to thoroughly go throw your changes.
But I already saw that you claimed example number 11 for the sum model after I did the same for the plurigaussian fields :-D Do we just keep adding new examples in an ascending order according to the time a PR is merged?

[MuellerSeb]

First come, first serve :-D