Enhancement: unbounded models

[MuellerSeb]

Unbounded variogram models like the power-law model can't be expressed by a covariance model, since they don't have a finite sill.

We need another class next to CovModel (maybe UnboundedModel), that provides only a variogram method. Only parameter in common with CovModel is then nugget (and maybe rescale).

Possible models are:

• UnboundedLinear
• PowerLaw (Webster 2007)
• Schlather model: https://onlinelibrary.wiley.com/doi/full/10.1002/sta4.134

Random field generation is currently depending on the spectral density of a model derived from its covariance function. Thus, we would need to implement new random field generators like turning bands or sequential gaussian for these models.

For Kriging, these models could be used immediately, but we need #191 for it to work.

[LSchueler]

Sounds exciting! Hello @OpenSourceCommunity, anybody interested? ;-)

This could be a nice master thesis.