Move gstatrandomfield out of spatialparams folder

The material/spatialparams folder is about to be dissolved with most header moved somewhere else. gstatrandomfield could be just moved one level up. It's only a helper to be used in spatial params (or elsewhere) but not a SpatialParams class definition.

Or deprecate in favor of something like https://gitlab.dune-project.org/oklein/dune-randomfield?