New Class for Conditioned Spatial Random Fields

[MuellerSeb]

Closes: #128, #126

This PR adds a new class for conditioned random fields CondSRF.

This class replaces the conditioning feature of the SRF class, which was limited to Ordinary and Simple kriging.

CondSRF behaves similar to the SRF class, but instead of a covariance model, it takes a kriging class as input. With this kriging class, all conditioning related settings are defined.

During refactoring the RandMeth class, a switch to select the sampling strategy was added.

Note

This change breaks backward compatibility, since conditioned random fields are no longer provided by the SRF class.

[MuellerSeb]

@LSchueler this is ready for review. ☺️

[LSchueler]

Wow, the SRF class is looking soo much nicer now!

Really nice work!

I suggest to import gstools.field.base.Field to gstools.field. The import in one of the examples looks a but ugly.

I just made a few small remarks in the code. I think after you have addressed we are ready to go!!

[LSchueler]

:-) Found another typo ;-)

But anyway, here is a big thumbs up!